I can't help but believe that you don't have the QS8's "seated" completely in the brackets. If I remember correctly, the tabs overlap or interlock by at least 1/2". Even if there was not any friction holding them together, for them to actually bounce out of the bracket, you'd have to be moving a pretty heavy speaker straight up by a significant distance.
If you can move them more than a fraction of an inch sideways when they are on the wall, you don't have them fully inserted into the brackets.
Since, when they are in the brackets, the only direction they can move is "up", couldn't you add a mechanical "stop" on top of the speaker - something simple like a nail or a screw or a little L-bracket or something.
Wish I could experience your subwoofer. Good luck.
