2 possible reasons. Your rear calipers are sticking for a mechanical reason (bad calipers) or the hoses between the metal brake lines and the calipers are collapsed (which will cause the calipers to stick). 

Understand, under hydraulic pressure when the brakes are applied it can overcome a "collapsed" flexible brake line.

But when that hydraulic pressure is released a collapsed flexible brake line won't allow the pressure to be released. 

The first reason is easy enough to tell by jacking it up and spinning the wheel. Is it locked up? Is there resistance. Or does it spin freely?

And to drill down on the second reason, If the wheel doesn't spin easily, simply loosen the banjo bolt on the caliper and see if brake fluid drips out and then if the wheel spins freely. That will tell you if the problem is or isn't with the flexible hose between the steel brake lines and the calipers because a collapsed brake line won't continuously drip brake fluid if not under pressure and loosening the banjo bolt will release the pressure on the caliper

Please use the length-limited subject line for a short description, not your full topic. Thank you. (Title edited.)

Is your parking brake sticking?