The ball was thrown slightly behind Brown forcing him to go back toward the ball to try and make the catch. This contributed to the DB hitting him a tad early. The DB did appear to be playing the ball. It’s one of those that could’ve been called. The fact that Brown had to turn back toward the ball and the DB to try and make the catch is probably why they didn’t call a PI. With Brown turning back toward the ball It appeared he may have initiated the contact.
Defenders can't run over receivers even if they have their head turned.
The ball is several feet away when the defender hits the receiver.
Brown slowed up but he was not moving back towards the QB or defender.
If the defender is running stride for stride with the WR and there are only a few inches of space when the WR slows up, then it could be debatable if contact happened just as the ball arrived.
In this play the defender has plenty of time and space to avoid contact and the contact occurs well before the ball arrives.
The quality of the throw is not relevant unless it is un-catchable.