Could have been rigor mortis. My HS biology teacher saw a corpse sit up. She was “surprised,” to put it mildly.
I saw a prank video, where a guy hid himself underneath a table that had a corpse that was about to be examined by a medical examiner. The guy ran an electrical current through the corpse, causing it to raise its right arm suddenly.
Yeah, the ME ran out of the room.
i have heard of this also.
In his mis-spent youth, a friend of mine was an orderly at a certain Cambridge hospital. They had a hazing procedure they deployed against new hires. The new orderly would be taken on a tour of all the hospital's facilities, ending at the morgue. A corpse under a sheet would be wheeled out of the fridge. Initially dead to the world, it would suddenly sit bolt-upright, turn its gaze upon the new hire, and jump up and chase him!