Why did Chris Pronger allow Dustin Byfuglien to beat him up so much last game?

He was constantly slamming into Pronger, and Pronger didn't really retaliate at all... He got punk'd... I know Chris Pronger isn't the biggest weight wise, but he usually is the guy that doesn't care if you weigh 40 or 50 more pounds then him and he'll take you on.