Snapped!?

Do people really simply snap and get violent? Seemingly calm, rationale people living in the suburbs engaging in violent crimes against their children or spouses. My opinion is that people do not simply "snap" rather there is ongoing stress or trauma; or there is another mental health issue that lays dormant or goes untreated until a severe stressor (i.e. infidelity, loss of retirement plan) causes a person to act out the violent behavior. This is not an excuse, they are still responsible for their actions. Saying someone "snapped" is a way for people to explain something that has no rational or acceptable explanation.