I think in 2 parts, he's booted the ball away for a goal kick and looks like he's going down before the contact. One of the replays looked more conclusive than others, he's falling into the defender. From the back it looks like the defender catches him with his right foot, from another angle it doesn't look like he touches him. Odd one. Don't think any replay looks conclusive so probably should stick to the original decision, Ref must've thought he fell into him.
In addition, McKennie seems to have put on some pudding.