You could call this a couple of different things. If the ball was in the air (which it does not appear to be) I would call it pass interference and be done with it.
Before the pass, you could have an illegal block in the back (yes the defense can be penalized for that), or a "blind side block." personal foul. this does not cross the line into the flagrant category in my opinion (and that is a judgement call). You can argue these types of call all day, but at the end of the day it is up to the individual official who is making the call. Some things are still truly subjective and "in the eye of the beholder."
I would be inclined to say this is a block in the back.
