I've heard from a few who have done it, and it seems to be basic questions that you would have researched yourself anyway. Like how many fr bases/pilots/planes, whats a ndb/vor, how does tcas work, how does the AI work...
It's not something to stress about if you've prep'd yourself for the tech interview.
They do it now because they can. Every Tom, Dick and Harry are applying to them. Might as well be choosy!