First: it is circular, because the self-selection needs to be kickstarted somehow. There needs to be a reason that liberals are more likely to see academia as a good environment to them that isn't "there are more liberals than conservatives in academia." It had to get that way somehow. Note that that could have still been self-selection but the motivation to get from no apparent bias to professors are all liberal can't be that professors are all liberal.
So, why is it that academia developed into a place where liberals gather? I haven't the resources (time or physical) to figure this out myself--I certainly do have a few thoughts on the matter--but this isn't a new phenomenon (some aspects are but not the whole liberal vs. conservative thought in educational settings).
I should also note that while this has some bearing on US politics, that isn't necessarily a big part of it. Particularly today, the political side of liberal vs. conservative doesn't really register to me as an intellectual debate. The two political sides are far more divided along emotional and group lines, though, yes, one side is far more welcoming of intellect/reason/facts/science/thought/... than the other.