Brakes Squeak When Backing Up

[backwoods lex]

Hey guys. Sometimes when I back up my car I hear the brakes squeak. It sounds like when the pads are worn and the indicator makes noise, but I pulled off all four wheels and the pads all have a lot of life left on them. It never happens when traveling forward, but always happens when I first back out of a driveway or parking spot. It does not happen after regular driving. Only when you first get in. My rotors are warped though. The car vibrates a lot when braking at highway speeds. I'll replace those by the end of the summer. Thanks for any help.