Definition of boil over:
(v) :
(intransitive, idiomatic, of anger, etc.) To reach the point where aggressive action is taken.
(v) :
(intransitive) To act aggressively angry.
(v) :
(intransitive) To boil to such an extent as to overflow its container.
(v) :
(intransitive, of horse(s)) To buck aggressively.