I don't think one can come up with a generalizing theory that would be very useful in this kind of question. You would need to look at a lot of cases to look for any trends, and of course it would be hard to do any kind of honest assessment of who started one way or ended up another. (There are also many cases of people who start their careers very far to the left, then switch to very far to the right, like Ronald Radosh).

What I do think one can say, though, is that history is almost inherently "political" in some sense, because it is an account about how the world came to be how it is. That "world" might be very small-scale ("how French botanists came to accept binomial nomenclature") or very large-scale ("how Western Civilization became so politically, militarily, and economically dominant"), and that scale impacts how fiercely these claims to the past are fought (the stakes are higher). But if it is something that people care about, then the story of why it is the way it is — and how you tell that story (a continual progression? a rise and fall? a downfall? a chaos?) — will matter to people today, because we base our identities and ideas about the future in part on a vision of the past.

So it would be very surprising if you did not find historians who had very strong political beliefs of all quarters. And it would be very surprising if people didn't ultimately have some kind of dynamic, reinforcing view regarding their history and their politics (it is very hard to make sure you do not end up favoring theories and evidence that just fits into your general worldview — it takes active work to avoid it).

This is separate from whatever processes may exist that encourage concentrations of specific political leanings in certain academic fields (which vary by field, and are complex).