The guy is 6'4" and 230 lbs. He's not the quickest, most nimble guy out there. If you look at the play, the way he was running his route, and where the ball was placed it would have been extremely hard to keep his entire body from going out of bounds. In short, he lost his balance.
He lost his balance because he was looking back at the DB and not looking where he was going and making a bee line to the end zone. It caused him to drift closer to the side line. He wanted to see the DB chasing him.