Sentence:  But as the half progressed, Liverpool's pressure and high-tempo passing game increased United's frustration and it threatened to <WSD>boil over</WSD> on the stroke of half-time when Van Persie, who had already been booked, was involved in angry verbal exchanges with several Liverpool players, particularly Gerrard. 
Meaning of the word marked with <WSD> tags:  To reach the point where aggressive action is taken.