Question for Football People

[gonzai]

The wide receiver catches the ball cleanly, turns and runs a dozen yards before being brought down. Pass interference is called on the catch.

Question: if it's a clean catch and the guy runs with the ball, how there be pass interference? If there was interference - big if - clearly it neither stopped nor slowed down the receiver, so how can it be interference? 

I'm baffled here. (It happened in the Ravens/Bengals game today.)