I've never heard of such that thing happening to a mount. I sorta sounds like the skull base has separated from the forms wooden panel underneath, and pushing up. But not sure why. By any chance is the mount near a source of moisture. Once it was dried it shouldn't have ever moved. If moisture has gotten to it over the yrs its possible the hide can move, and the board holding the skull plate has rotted. Thats only thing I can think of. A pic of it would be easier to tell.