That looked to me like a flop by Zeller.
You are looking at it with your eyes right?
My eyes see a flop too.
You have 2 guys about the same height and weight (7'1" 250lb vs 7'0" 250lb). One takes a step and lowers a shoulder causing the other to fall off his feet completely. In no other place in the world (except soccer/football) would a physically fit person whose expecting contact be knocked off his feet by that.
Zeller had position, didn't move and was outside the restricted area. Mozgov dropped a shoulder and charged into him with full force.
What difference does it make if Zeller fell naturally or flopped? According to the rule books that's a foul regardless.
Oh and the argument of those guys having similar weights really is a moot point. I'm a big guy (about 6'0" and 260lbs) and I can guarantee you that if another 6'0" and 260 pound guy dropped a shoulder and ran full force in to my chest, I'd have a pretty hard time holding my ground without at least getting pushed back a good 20 inches...and that's if I braced myself as hard as I possibly could. If I was just standing my ground (and not bracing with full force) it's not at all unfathomable that I'd fall back like Zeller did.
At their height you have a higher center of gravity, so you have even more chance of falling.
If Zeller's drop was exaggerated, it wouldn't have been by much.
I consider myself a pretty fair guy, and I'm happy to defend the officials when there's a call that would be legitimately tough to call in full speed action...but there is no way you could NOT call that charge. The guy absolutely steamrolled him.