loose cannon

ot noun

1. (by extension) An uncontrolled or unpredictable person who causes damage to their own team, faction, political party, etc. Jack is considered a loose cannon due to his volatile personality and his track record of being unable to maintain his composure.
2. A cannon that breaks loose from its moorings on a ship during battle or storm, which has the potential to cause serious damage to the ship and her crew.

fe’l verb

1. To behave in a way such as to cause damage to one's own faction, political party etc.