They played off him as a non-shooter, going under picks consistently. Ideally your big man then corrals him from a straight line drive, which Dirk/Tyson didn't do at all.
Rondo could have done better of course, but when you're getting picked at the three point line and the big can't do anything to impede him at all what can you say? Just poor defensive effort all around, but Atlanta will do that to you.