Don't make long run with this car. After a short ride, touch the rear wheels (not the discs) prudendly as you know they can be really hot. If it's the case of one, your brake is sized (it will not be electrical then).
I see 2 things :
The cylinder in the galiper wasn't adjusted correctly (we have to rotate the brake cylinder to gain space for new pads). Some will just put the new pads without rotate it and they will be very tight then.
The rear brake bias valve can fail (normally, it will give more rear brake power if the car is loaded). Mine stopped to give more brake at one wheel. The starts progressively to steer to one side when braking even with new brakes all around and nothing in the trunk... I'm wondering if this valve can allow the fluid to pass to the rear wheels but will not allow fluid to return.
Keep us informed,