If you adjust the take up on the brake pedal to the master cylinder too much, heat expansion of the metal linkages and brake fluid will cause the brakes to engage. It just happened to me after I replaced the pedal bushings and had to reinstall the linkage. I adjusted it a little too tight and after the 15 minute ride to the feed store, the brakes were dragging on the ride home. It was all 4 corners for me, all the drums were very hot and I could smell burning brakes. I just backed off the pedal adjustment and the problem was gone.
Your issue could be a combination of the pedal adjustment and maybe the shoe adjustment if you're only seeing it on the rear brakes. I'm sure you'll get it after some trial and error. At least that means you get to drive it bunch!
