The phrase "to boil over" means to become very angry or upset, and express that emotion in an intense way. It is often used figuratively when describing a situation where emotions have reached their peak and are spilling out into action. For example, if someone is cooking something on the stove and it reaches boiling point, it will overflow and "boil over" onto the surface below. Similarly, when people's emotions reach this level of intensity, they may say or do things that they wouldn't normally do, as their feelings are spilling out in an uncontrollable way.