Right, I believe the grid fins are the primary roll control method (the other being friction and secondarily tilting the ship and making a precession around center of mass). If this force was lost while the control loop was expecting it to work things can go bad I guess.

Also attitude control. The stage isn't going to be much of a glider no matter what you do with it, but at terminal velocity, it has nontrivial lift and drag, and changing those will change the trajectory.