Quiet shims creeping out from between pad and piston

I heard a scraping noise from the front right wheel area on my 90 Accord. I figured the brake pad had worn down to the squeeker so bought a new set of pads. When I got the wheel off I could see that the quiet shims had crept out about 3/8 in so that they would drag on the disk. That part was pretty chewed up. I ordered all 3 shims, not local stock. I don't understand how this can happen. The shims are secured by ears which slip over the pad. I replaced the pads myself about 40K miles ago. You would think that an assembly error would have shown up a long time ago. Any idea what went wrong?? Bob