Mental mistakes are things like the BYU offensive staff going completely passive in the second half and throwing away a 20+ point lead. Watching the defense in the second half fail to make any adjustments to a Ute offense that is rolling all over them.
Most of the mental mistakes seem tied back to the coaching staff. Give Kyle credit, in the chess game of football tactics he's made Bronco and Kalani look really bad for a long, long time.