Brake pads will sometimes squeak when rolled backwards, simply because they are rarely used in that direction.
The pad sticks to the rotor momentarily, releases, sticks, releases, etc. and does it fast enough to make a squeak.
I really wouldn't worry about it.
If you absolutely have to do something about it, remove and clean the rear caliper, bleed fresh DOT4 through the rear brake system, and fit new pads.