I didn't watch the game but from what I've read this morning the Bears coaching staff couldn't adjust to the changes GB threw at them.
And it sounds like GB was not prepared for Mack at all.
I think they took Mack out of the game with their quick passes. He was winded as well, according the observations of analysts.
I think the Bears went into the Jim Caldwell shell of "playing not to lose" trying to protect a 20-point lead. I've seen that way too many times with the Lions.