Every year there seems to be a good number of transfers from that league going to P5 programs....Harvard, Yale seem to have the most. I have no numbers just perception.
Grad students can't participate in athletics so if for any reason a player redshirted, he'd have to leave to use up his eligibility. That's one reason. Another is that Ivy League teams are recruiting at a higher level these days so these guys are more in demand and have more options if they want to transfer to a better basketball school. At least that's my thought.
NJ product Bryce Aiken is a grad transfer who played at Harvard and just committed today to Seton Hall as expected. Will be one of the stars of the Big East to replace Myles Powell and Quincy McKnight.