yeah, probably proximity to the medium -- messes with transpiration, which is a primary control for the flow of water in the plant and therefore nutes to the leaf. At night i bet they droop down and rest on it all night.
LEave'em be.. they don't hurt you.. if the plant sheds them, they'll be sucked down and put to good use in the meantime. Always let a plant suck'em down before removing, if possible. Some extreme context might warrant early removal.