This is the 9449th most frequent Japanese word.
突発
"突発" means "sudden occurrence" or "unexpected outbreak."
The word '突発的' (toppatsuteki) is used here as an adjective meaning 'sudden' or 'unexpected', describing the actions.
彼の突発的な行動が驚きを呼んだ。
His sudden actions caused surprise.
Here, '突発事故' (toppatsu jiko) combines '突発' with '事故', meaning an unexpected accident.
突発事故が発生しました。
A sudden accident occurred.
In this sentence, '突発' (toppatsu) is used as a noun meaning 'occurrence' or 'sudden happening'.