Michigan also seemed to waitlist me and several of the people I knew at MIT. If they know they're the backup school, and they think you're going to get into your first choice, it hurts their acceptance rate to accept you outright.

Though, maybe it's just confirmation bias and ego. Maybe there was something in my application that gave Michigan pause but seemed fine to MIT. At that time, Michigan did ask which other schools to which you were applying. I'm not sure if that's still the case.