It's been a while I haven't seen Rondo dunk. He used to dunk a lot on 2008-2009 season. Last season, he didn't attempt or do it. This was the recent I've seen him dunk against Bosh (Heat).
But still he has hops to do that. And he's been working out all Summer in the Celtics Facility.
Rondo still has hops | Red's Army - The Voice of Boston Celtics Fans
I think maybe this he don't want to dunk, again. He dunks hard and gets strong physical contact.