Many pads (maybe even "most") will eventually come off at the corners or sides, maybe first after many years, maybe fast sometimes - one never knows...
It will start at the corner(s) of the pad usually.
No need to take the whole pad off, or sand anything - just remove if there is too much glue remains under the "flap".
I just use a good waterproof contact cement, put it on both sides and let it dry 15 minutes while the corner is wide open (use a match to keep it open) - and then it can be closed firmly and you are ready to go "forever" again now.
Works like a charm and doesnt take long...
Even a bit excess glue outside the pad, can just be removed with your fingers, so no biggie.