The most common cause is a siezed brake caliper. To confirm, drive it, raise the wheel and see if it is hard to turn. If the wheel is hard to rotate, like the brakes are applied, loosen the bleeder screw. If the brakes stay applied, the caliper is the likely culprit. If they release, there is a hydraulic problem in the system. Common causes of hydraulic problems include brake hoses and master cylinder.