It's not always a negative. People who are heterozygous with sickle cell disease are immune to, I think, malaria. Still a disorder, but it can be beneficial.
Not every disorder is caused by mutation. Some disorders are caused by diet or social or physical environments.
I would still call sickle cell anemia a mostly negative mutation though. But yea, it does have advantages.
Almost all disorders have underlying genetic causes. Especially the ones mentioned here.