It's just plain poor technique. The play was a hot read requiring T-Will to sit in the zone against the blitz. It's not a pass requiring to be super accurate because it's not a route designed to get a lot of YAC as the receiver stops faces the QB. It's simply designed for the QB to get the ball to the WR before the blitz gets to the QB. T-Will's technique was for a pass to hit him in the chest which is fine if it hits you in the chest.
Irvin wasn't a body catcher either. There are times when you catch the ball with your body and there are times when you pluck it. You can catch slants with your body, but the issue is you are likely to lose some valuable YAC if you do so. With other routes, plucking the ball increases the WR's catch radius.
Williams just never adjusted his hands properly to the pass and that's why it went off both of his hands.
What's frustrating to watch is that Butler's catching technique has greatly improved. He's not always consistent, but at least he has made strides. He's not the blocker that T-Will is, but he's still a good blocker. And he gets open more often than T-Will does. Yet, we still insist on making T-Will the starter.
YR