When he heard the unfair decision, his anger boiled over immediately.

boiled 是动词 boil 的过去式，boil 本意是"煮沸、沸腾"。 boil over 是一个短语动词，字面意思是"液体沸腾溢出"，引申为"情绪突然爆发"的意思。这里用 boiling over 比喻愤怒情绪像沸水一样突然爆发出来。