Really good coverage up till the part where he grabbed the receiver. Come on man, get serious. It wasn't bear-hug level PI but the receiver's shoulder and jersey were tugged as he was running full speed for the endzone. At those speeds even that much impediment can throw a guy off course. It was PI, read the rulebook.
NFL Rules Digest: Pass Interference
"Actions that constitute defensive pass interference include but are not limited to:
(a) Contact by a defender who is not playing the ball and such contact restricts the receiver’s opportunity to make the catch.
(b) Playing through the back of a receiver in an attempt to make a play on the ball.
(c) Grabbing a receiver’s arm(s) in such a manner that restricts his opportunity to catch a pass.
(d) Extending an arm across the body of a receiver thus restricting his ability to catch a pass, regardless of whether the defender is playing the ball.
(e) Cutting off the path of a receiver by making contact with him without playing the ball.
(f) Hooking a receiver in an attempt to get to the ball in such a manner that it causes the receiver’s body to turn prior to the ball arriving. "
I saw both A and C on that play. and when the ref is right there, sees it, and throws the flag without hesitation, I think the PI is pretty clear and obvious.