It is caused by the front feet staying on the ground too long, thus allowing the hinds to catch up. It can be caused by many factors. This is actually a rider problem. You need to get your horse more engaged behind and off the forehand. Consult your riding instructor for details.
Forging can also be a subtle sign of unsoundness, especially if it only happens on one side.
In this case the horse is leaving the front foot on the ground longer in order to protect the diagonal hind. This problem will usually be accompanied by an unevenness while riding. The horse may not bend one away as well as the other. He may not want to take a canter lead. He may shorten his stride going one way on a circle but not the other.