When you say that they fly off the backing plate, do you mean pad and velcro, or do you mean delamination as in the pad flies but the velcro stays put?
I think I was a bit quick in reading your post. If they are simply flying off and not holding...That's a different problem. It could be a bad batch. The age of the pads could have an effect, as well.
I have experienced this problem with the PC 4" pads but never w/ the larger pads on the PC and never w/ the rotary. Not sure what caused the 2x it happened to me but it's relatively rare so I don't worry about it.