What would make my son have a relapse in his drug addiction?

No particular reason. Addictive brain gets wired differently after addiction - learn more at http://learn.Genetics.Utah.Edu/content/addiction/.
Its common. Relapse is part of the disease of addiction, majority of people relapse after first trying to get sober. Right after leaving rehab is a very vulnerable time, from a biologic standpoint triggers from people, places or things cause powerful urges. Relapse is not failure, it can be a good learning experiance, a trained counselor can help along with meetings, what's important is to remain willing.