I have had that happen to me (and my wife) a few times....quite annoying when your not expectiong it
It's also happened on the couch while watching tv. I actually felt like I was going to fall over as the weight was considerable. We could actually see a grove on the cushion....freaked us out a little.
Cant tell you exactly what it is as i am no expert but I can relate as i have lived it just the same.
Thanks for sharing OP. S+F



