"With Greenlaw still recovering for the first 13 weeks, Campbell started 12 games and played 90% of the defensive snaps with 79 tackles, including three for loss, and two passes defended.

Greenlaw's return Thursday night offered an immediate spark for a Niners defense that held the Rams without a first down through the first quarter and just three points in the first half. But Greenlaw began dealing with some knee soreness in the third quarter, which is when the Niners asked Campbell to enter the game.

According to Shanahan, Campbell actually had been asked to enter the game in the second quarter when strongside linebacker Dee Winters departed with a neck injury. Demetrius Flannigan-Fowles had to step in for Winters after Campbell told the coaching staff he didn't want to play. He headed into the locker room in the fourth quarter — a move that Shanahan said the linebacker made on his own."

So it doesn't directly say why he wouldn't play, but sounds to me like he was upset that he wasn't starting when Greenlaw came back from injury.