usually pads, mostly high performance pads, will have the leading edge shimmed (bottom edge of pad when rolling forward) off so it's not a squared edge. Having a squared edge sometimes causes that part of the pad to hit before any other part, causing the pad to "bounce" or chatter on the rotor. It is actually a vibration, bouncing really fast, and that chattering vibration causes a squeaking sound. If you want, you could pull your pads and see if they are in fact shimmed on one edge. If not, don't worry about it but then you could try filing down the top, trailing end, corner pad which is the leading edge when reversing. That should solve the problem, unless the pads are already shaped appropriately. Good luck, hope that helps a little.

-Andy