Q: With new rear brakes, car still won't spin on its own

There is a problem with the rear brakes on my car. Even with the following done, it still will not spin on its own: car jacked up, E brake released, wheels off, brake pads replaced, and caliper piston pushed back (not screwed back). I put the wheels back on and lowered the car, shifted into drive, pressed on the gas pedal and the car’s rear end sank. The wheels were spinning, but they were making that kind of sound when the calipers are pushing on the rotor and not allowing it to freely spin. I’m wondering if the brake pads might be the incorrect thickness or perhaps I did not compress the caliper enough. Instead of screwing it back into itself, I had pushed it in with a clamp. I got so frustrated I put the old brakes back on and the rear wheels now spin freely!! Why did the new brakes cause the wheels to lock up and the re-installed old brakes are allowing them to freely spin?