How do crazy people go crazy?

It depends on what you mean by crazy. Ordinary people can sometimes act a little crazy, usually out of frustration at not being able to exert normal control over something or something happening one too many times that they can't seem to overcome or avoid.

If you mean crazy as in eccentric, some people go one step farther in their annoyance, frustration, and otherwise being unable to change something or things that really upset them and eventually develop various coping strategies that make them seem odd or unusual to others.

If you mean crazy as in mental illness, there are a few different reasons a person developing these conditions and they are conditions that the sufferer has much less or no ability to control or alleviate themselves and require professional intervention. Reasons for these are dysfunction or chemical imbalance of the brain, a traumatic occurrence or occurrences in their lives that are beyond a persons ability to cope, or suffering damage to the brain itself.