Reminds me of a cat I once had who did something similar: She'd fall asleep on the windowsill...and then roll over & fall off! I'd be sitting down, quietly reading in total silence...then there'd be this loud "THUMP!" when she'd hit the floor! (Relax, people-she didn't get hurt--she was "well-padded", IYKWIM...) She never learnt, either-she did the same thing over & over throughout her long life...