If the long straight part of the hook is bent or bowed enough (or if the treadle slot has warped) the hooks can get stuck in the treadle slot. Then, when the treadle is released (traveling upward), the hook moves with it (instead of sliding in the slot) and the hook itself is lifted up off the lam.
Straighten hooks and wax them to be sure they can move in the treadle slots freely.
