Hold up. Watch this video from a different angle. This is not a flop!
Shai looked back at Champagnie to know where to position himself for the box out, like hoopers have been taught to do since grade school. Champagnie’s 2 hand push to Shai’s back dislodged him from his rebound position, and affected his ability to compete for the rebound.
This is a LBF, not a flop!
The referees were correct in denying Mitch Johnson that challenge. The rule is very specific: he needs to first call timeout, then challenge. He never signaled for the timeout before signaling for the challenge.