Overlapping collision boxes result in bizarre physics bugs
When an actor's collision box overlaps with that of a static, the actor is moved up into the air and moves up and down in a rapid and erratic manner. This absolutely does not occur in the vanilla engine. The behavior in question is most easily observed with mannequins. If you download and install v1.3 of Darknut's Dwemer Home, found at https://www.nexusmods.com/morrowind/mods/45715? and coc to Kazara, Living Quarters, then head up the stairs, you can see the glitchy behavior on the mannequins near the fish tank. You may also be able to see it on the fish in the tank.