Watson would get three free throws on the possession and the ball, but turned it over, and the Heat converted it into a dunk. But a clear frustration foul. While it may not look like much, pushing a player when he's in mid-air is considered extremely dangerous, and that went a long way towards that call.
Chalmers was awarded a common foul and a flagrant foul, but in a weird rule, a player can't score four free throws on a play without a three-point attempt. If Watson had missed one of the free throws he would have gotten another shot.
It's probably not the last flagrant foul that will be called in this series.