There are a few different things that can cause your brake system to drag. The most common culprit is dirt and debris build-up on the brake pads. This can happen if you don't regularly clean your brakes or if you ride in muddy or dirty conditions often. Another possibility is that one of the caliper pistons is sticking, which will prevent the pad from retracting fully after it's applied. This can be caused by a leak in the hydraulic system or simply by age and wear. Whatever the cause, brake dragging can be dangerous as it can lead to overheating and failure of the brakes themselves. If you notice your brakes dragging, have a qualified mechanic check it out as soon as possible.