Uneven disc brake wear can be caused by various problems, and these problems affect other types of brakes as well. The usual suspect for uneven brake wear is lubrication, or lack thereof. Disc brakes use bolts, or pins as guides for the caliper to travel on. If they are dirty, or have not been lubricated, the caliper will not be able to extend and retract with ease. This means that one or more areas of the brake pad(s) are receiving less pressure than other areas. This causes uneven brake wear, and can result in having to prematurely replace the brakes.
For more info, see How to Reduce Brake Wear