Seven years ago I wrote a post explaining why I think a common theory among biblical scholars is both against our best evidence and unnecessary in order to explain a few puzzling features of the texts we have. The puzzling features are as follows:
When Aaron dies in the Torah, it says his son Eleazar takes over the high priestly position, and then Eleazar's son Phinehas inherits that role when Eleazar dies. Yet the line of Eleazar does not seem to maintain that position by the time of Samuel. Eli seems to be occupying a high priestly role, and he's descended from Eleazar's brother Ithamar. Yet the biblical texts do report of the line of Eleazar being preserved, notably in a man named Zadok, whom David seems to elevate to a high priestly role of seemingly equal authority with a continuing high priestly descendant of Ithamar. It's only when that man betrays David that we seem to have a return to one high priest.
The common scholarly theory takes the texts to be unreliable reports of events. There's no direct evidence that Zadok was anything other than a Levite descended from Aaron through Eleazar. There's no direct evidence that the Eleazar line was invented wholesale in the Torah in order to retcon Zadok as a more legitimate priest than Ithamar's by-then-disgraced servants who had sided with the coup against David. But the suspicion because of the puzzling facts of the previous paragraph has somehow become unquestioned and even is presented as obvious by a lot of biblical scholars, when there are several other explanations of why the text reports what it does, none of them less likely to my mind than the suspicious explanation. I give two in that post seven years ago, and a third occurred to me this morning.
One possibility from the previous post is that the descendants of Eleazar had forsaken their responsibilities during the time of the Judges, which is entirely fitting with how Israel is described during that time, and the descendants of Ithamar were left to run the operation of the tabernacle and early pre-Solomonic temple structures (like the one we see in the early chapters of I Samuel).
The other possibility from the earlier post is just a decentralization of worship, not really being faithful to the tabernacle set up in the Torah (which would also fit with what the book of Judges tells us of that period). In this second case, Phinehas' descendants might still have been operating as priests, and indeed may even have considered themselves high priests, but other priests were operating in other locations, contrary to Torah specifications, and in each location someone was functioning like a high priest for that location.
The third explanation that occurred to me this morning is that there was a pattern for selecting the high priest that didn't consistently follow our expected rules of succession. Perhaps Phinehas' selection of high priest to succeed Eleazar has wrongly suggested to us that it would always continue as father to eldest living son. But perhaps instead the rule was eldest living male descendant of Aaron. If Ithamar died before Eleazar, then Phinehas might well have been the oldest male descendant at that time, as the eldest son of the eldest son of Aaron who had children (the oldest two seem to have died without children, or else their entire lines were disqualified for their fathers' sins). But the next high priest might have been a younger brother of Phinehas or an uncle or cousin from the Ithamar line. And this might not have had to have been a rule adopted at the outset. It could even have been a modification implemented later on, whether legitimately or not. The Torah doesn't ever specify, from what I can remember, how the high priest would be chosen. It might have been by Urim and Thummim or something, in which case the high priest could even be the youngest priest of age.
I think any of these explanations could be true, and all of them seem more likely to me than the possibility that the texts were all fabricated in order to explain a non-Aaronic family becoming pretenders to the Aaronic line. In a culture with such a high emphasis on geneaology, especially as evidenced in the narratives of Ezra, Nehemiah, and Chronicles to establish credentials beyond a shadow of doubt before someone could serve as a priest or Levite, I find it incredibly unlikely that someone could craft such a massive conspiracy that would rewrite all these texts that would have been so known and loved by the faithful Israelites who returned from exile. There's such an emphasis in those books on the careful study of the Torah that the conspiracy theory has to be far less likely by any reasonable weighing of the alternatives than any of the three possibilities that I've proposed.
There are a few other facts that might help us sort through the options. I'll look at just one, because it's sometimes used as an argument against the kinds of reconstruction I'm putting forward here. The texts from Samuel and Kings never refer to any of these priests as high priest. The conspiracy theory takes that to signal that the Samuel and Kings texts were earlier, and the language of high priest wasn't developed until much later and inserted into the Torah along with the concocted genealogies and stories that either invent Eleazar altogether or fabricate his genealogical connection with Zadok. But there are perfectly reasonable explanations for why this would be that completely fits with the texts we have without the massive reinterpretation required by the conspiracy theory. Taking the texts at face value, we have a scenario where there's a high-priestly position set up, and at least on the first two explanations, something goes wrong with that scenario. Someone is functioning as high priest who, ideally, shouldn't be.
1. On the scenario of the fallen line of Eleazar, the rightful high priest isn't even functioning as a priest. So the person who takes on that role, knowing that he's not really the high priest, refuses to call himself high priest. Then that becomes the habit, and when David restores the Eleazar line with Zadok he maintains two leading priests until Zadok takes over fully by himself, and even then we have the habit of simply referring to him as the priest (as opposed to a priest).
2. On the decentralization hypothesis, the leading priests in each location might have been hesitant to call themselves the high priest, and it became habit to refer to even the rightful high priest as the priest, because there wasn't a lot to distinguish him from the other chief priests in other locations.
3. On the alternate method of deciding the high priest, there may actually have been a continuing high priest, who was called high priest all along, but the rightful high priest may have been faithful to Saul and not David. David, perhaps too loyal to the man who had been functioning as priest under him, was willing to bring Zadok on as a parallel leading priest in order to comply with some of the Torah regulations, but did not make him (or at least did not call him) high priest until he was the sole leading priest.
I don't think we have enough information to sort through which of these three might be more likely with respect to each other, but they all seem more likely to me than the conspiracy theory that many scholars seem to have adopted as the only obvious reconstruction of historical events. And the mere possibility of three coherent reconstructions that could easily be imagined to have happened should give us pause at any attempt to reconstruct the historical scenario in a way that actually conflicts with the only evidence we have about these events.