The accident was caused by a situation not unexpected or planned beforehand that will occur. When it happens, it produces a lot of subsequent damage. Sometimes it is serious damage, even death.
The accident was caused by the unplanned or unexpected before that happens. When this happens, it will cause. Damaged by many Sometimes a serious blow to death.
The accident was the situation without a plan or expected this to happen. When it happens, will cause substantial damage. Sometimes a serious harm to death.