Historically, a liminal space was just a place a where two areas meet, like a doorway, a gate, a crossroads, or even death. Somewhere a journey ends and another begins. I think Hecate, who was the goddess of crossroads and magic/illusion (and hellhounds) among 1000 other things, was in charge of liminal things. (Makes you wonder where Robert Johnson picked up a good chunk of the Hecate myth). I think Janus also played a role in controlling exits and entryways.

So, how does a rather normal Roman concept get turned into whatever liminality is now? The answer is that the idea was picked up first by Early Modern philosophers (I think Kant was first but he had a very unmystical definition of the liminal -- but I won't bore you with... Kant). Keep in mind that during this period the limit in mathematics was being developed.

It gradually regains something of its spiritual, Roman-like meaning as the 19th century progresses and also finds an appropriate home in anthropology to describe places and ideas like the Roman ones and in (Jungian I'm guessing) psychoanalysis where it's used metaphorically as turning point or a threshold between sanity and madness.

After that, the French philosophers (maybe from existentialism on -- existential literature, Satre and Beckett come to mind, have some sublime depictions of liminality) of the later 1900s, ran further with the psycho-metaphorical version. Besides its already-established connection with madness, it came to be, I guess, a sort of goal. I think the aim was to push the writer, reader, and message into a liminal space where cultural and other boundaries could be crossed with the hope of seeing things from an alternative perspective.

From there it trickled down to colleges and culture at large (again). Sorry for the ramble. I hope I got my facts correct.