Where did you disconnect them at? If it's at the caliper's, and you still are having problems, it's the calipers themselves. The only thing that I can think of about it happening mainly when it's in reverse, is that when slowing/stopping from reverse, the rear discs are doing more work than they do when moving forward. Same idea of the front brakes doing 70% of the work during regular forward motion, only it's not 70% on the rears in reverse.
