The NCAA didn't "mandate" it - SU threw it in as part of their preliminary self-imposed sanctions, as they tried to get out in front of the NCAA hammer fall.
My personal taek on all of this is:
JB was convinced by SU admins that he needed to fall on his sword due to the NCAA , but he refused to do so immediately, so then the "will retire in 3 years" thing got put out there.
Everybody was fairly OK with that. For a while.
Then JB got that 9 game suspension, and a real-life taste of what his world minus coaching Syracuse basketball would be like. Juli is on record stating he hated it, he was miserable.
Hop realizes JB doesn't really, really want to retire.
Hop doesn't want to be the guy who forces his mentor to retire before he's ready to.
Hop has a sick father on the other coast, so when the Washington job opens up, it neatly resolves all of those issues for Hop AND for JB:
Hop gets a P5 HC job, near his Dad, and JB gets to keep coaching for as long as he wants to.