loose cannon🔊

(nautical) A cannon that breaks loose from its moorings on a ship during battle or storm, with the potential to cause serious damage to the ship and her crew.

(idiomatic, by extension) An uncontrolled or unpredictable person who causes damage to their own team, faction, political party, etc.

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