One possibility is a problem with the residual pressure valve. They're installed whenever the master cylinders are lower than the brake caliper and, for disk brakes, supposed to retain 2 PSI in the lines. If that valve is sticking or defective it won't allow the pressure in the rear brake lines to drop after application of the brakes.
It will look like this picture (or very similar) and be installed inline on the line between the MC and the rear brakes.
I would think they're pretty simple and reliable, but perhaps some debris was in the system and lodged in the valve. I don't know if they're repairable, but they're relatively inexpensive and easy to replace, though you will need to bleed the rear brakes when you're done.