A "loose cannon" is a person who can't be controlled, and is liable to do totally unpredictable things. If you'd ever been in the well deck of a ship in stormy seas, with a piece of equipment that has come un-dogged, especially something like a cannon or a jeep or such, you would understand the term better. And you would understand why a real loose cannon can sink a ship just as fast as a metaphorical "loose cannon" can sink a career.