I think Rondo has been pushing the ball more this year than in the past. I think he also has been more willing to make his first pass earlier in the possession than in the past. Both pluses and credits to Rondo.
The "problem" is simply more pronounced in the 4th quarter which is not Rondo's fault, more accurately, just an exploitation of his weaknesses as a player. In the fourth quarter, there are more time outs, fouls seem to be called a little tighter, the opposing defenses seem to hunker down more, all contributing to a slower pace. There are also time at the end of games where the Celtics and Rondo in particular do slow it down intentionally. All teams just tend to this if the game is close.
The problem is that once the pace is slowed down, they can sag off Rondo, dare him to shoot, and the defense will be set making it harder for Rondo to make passes. Our offense grinds to a halt with Rondo dribbling around trying to create something. As I said, I don't blame Rondo for this per se, it is just what you get with Rondo based on his talents and weaknesses.