An accident is something that happens outside of intent.
An incident simply refers to something that has happened intentionally or not.
Wetting the bed is both an incident and [normally] an accident.
Throwing oneself in front of a train is an incident, not an accident.
Falling in front of a train is both an incident and an accident.
All accidents can be considered incidents, but all incidents cannot be considered accidents.
Incontinence is a medical condition, not an event. Therefore it can properly be called neither incidental nor accidental. But an instance of (an occurrence of) incontinence is both an incident (an event that occurred) and an accident (an unintentional event).