My guess: it's in a cool city, it's a well-known school (of sorts), it's not terribly difficult to get into. Thus, everybody wants to go there and thinks they have a shot. BUT, BU only has so many spots available...must my guess.
They have a few things that inflate their numbers, though. They have the accelerated medical program (for high school students) where well over 1000 kids apply for 20 spots. They also have a program for BU students where they can opt into the school after sophomore year. I don't know how many apply for that, but I think they offer 5 spots. If you were to take all of those applications and seats out, you'd be left with a much more normal looking number.
IMO BU jerks people around a lot more than other schools...they do everything in their power to make sure they don't give acceptances to people who won't matriculate. Just looking at the posts in this forum and from personal experience people complain about the BU waiting game more than other schools.