If you use the wrong entity type on some models, they will sometimes "fly" off somewhere and knock over other props (especially when you're trying to make things hanging on walls). I would change the type of phys_ entity you're using; prop_physics are usually ok but phys_dynamics and phys_static are also required to stop this. Play around a bit, place copies of your entities (all these different types) in different parts of your map and see if they fall over. They also tend to fall over if you sit them on top of other models...