What, specifically was the foul?

Kicked his foot out while shooting, trying to fool the ref into a call. Equivalent of flopping for a charge. And it's dangerous, Jrue could have wrecked his ankle.

No NBA player catches and shoots with their focus entirely on the rim while simultaneously calculating where the defender, who is leaping right at him, is going to land and decides to stick his foot out during the follow through of his shot. Come on, bro. Be ffr.