Here's the rule:
Rule 8, Section 5, Article 2 (Forward Pass, Backward Pass, Fumble)
Prohibited Acts by both teams while the ball is in the air. Acts that are pass interference include, but are not limited to:
(e) cutting off the path of an opponent by making contact with him, without playing the ball.
It seems to me that that is fairly open to interpretation. Worthy made contact with his defender and drove him into the other defender without playing the ball. Worthy did initiate the contact, which was clearly meant to impede the defender on that play. He wasn't trying to get around the defender. He was trying to "go through" him. That's why the ref called it.