They say it is a disorder. The things that looks disorderly are given a term of disorder. Scientists have given reasons like chemical imbalances and under-use or over-use of some sections of the brain, a root cause for Obsessive Compulsion found in some people. But in my opinion, having experiencing it first hand from the age of 8, it is not a disorder but an attempt of people like us to give an order in our daily lives.
It is something like conditioned by the mind over the years, to repeat some kind of actions that doesn’t look rational seeing from the eyes of our “perfect” outer world. But the people who goes through it have reasons to do so. The insecurity, distrust and self doubt give rise to checking locks, switches multiple times to appease the inner self that physical imperfect body has done its work properly and is not fooled by illusions, assumptions and distractions of the outer world. One more example could be, getting impacted by a violent show in television. And started worrying about the security of one self and the related family. As a result started repeating a special kind of prayer, at a specific time of the day, to the God, to save people from the hands of evils as portrayed in the show.
Hence, there are reasons beyond scientific generalizations and apathy, that to me, looks rational enough to be performed like rituals everyday and hate it to see the world calling it a disorder, when the person knows it is just Obsessive Compulsion to give an accord to the inner anxieties and provide inner peace.