94 deville rear brake issue

about 4-5 months ago i had my rear pads and rotors replaced by firestone. only reason i did this was cause i couldnt get off that bolt underneath the brake cable. anyway 2 days ago i noticed a grinding type noise in the rear drivers side wheel. i took it in cause the work was under warrenty. they replaced both rotors and pads for free. the drivers side pad was %100 worn. they said that the caliper had retracted?? they say if it happens again i will need new calipers which the labor will be under warrenty and the parts will run about $200. thats not so bad but im wondering if anybody knows what might be going on to cause such a problem.