This is a really odd one!! A pad breaking off it's backing would suggest massive overheating (ie caliper sticking on - due to sticky sliders, sticky pistons or sticking-pads). But you say it's ok? By the way you should make sure you never get normal mineral oil or grease on brake rubber seals as it can perish them, lubricate the pistons with brake fluid when you put them in the caliper.... MASSIVE wheel bearing failure might do this too in theory, but the wheel would be all over the place so you'd notice. Or crash. Stripped caliper bolt - tightening slightly then slipping "loose" again?