Most likely your rear brake hose needs to be replaced. As brake hoses wear, they swell up on the inside, constricting the flow of brake fluid. When you apply brake force, it's enough to press the pads against the disc. But when you release the brake, the constriction in the hose prevents the fluid from flowing back, and the pads stay pressed against the disc. You can't see this problem from the outside.
Your bike is not that old, but maybe the fluid was never changed and is contaminated including with moisture that brake fluid, that is hygroscopic, absorbs over time.
In any event, try cracking open the bleeder valve on the caliper with the bike on the center stand after checking to see whether applying the rear brake makes the brake bind up. If the wheel is free after opening up the bleed valve/nipple, that will confirm whether your issue is caused by a faulty brake hose.
If it is, I would replace all hoses, including the fronts, out of precaution. The rear hose costs about $30 original BMW part in Europe at least.
