Dude, the way BYU lined up, the official on that side had literally only one thing to watch in his assignment: The WR/CB matchup. So when it happened that obviously, the official really had no other option. He did not wait to see how the rest of the play went, he threw the flag. Maybe if it happened on the side with 3 WRs, it doesn't get called, but it happened where there was only that one matchup to be watched. Good call.