The last time an NBA official called an illegal screen on a member of the Boston Celtics, it was 2007. OK, slight exaggeration, but you get my point. The Celtics get away with more illegal screens than anyone, to the point where it's just become a consequence of playing them.
You can imagine everyone's surprise, then, when Kevin Garnett just got called for a foul on this play.
Let's be very clear here: that's a foul. Garnett didn't set a screen as much as he put his hands out and dislodged Andre Iguodala like a pulling guard clearing a running lane for Adrian Peterson. That said, you can understand why Garnett did that. NBA officials always swallow their whistles late in the game, and no team takes more advantage of that than the Celtics.
That said, it's still a foul and it was the right call. Doc Rivers' facial expression tells me he knows this too.