A WORD OF WARNING

HI all.
Went out yesterday in my 215AS , sea conditions were calm to slight and I was accompanied by my stout friend of some 20 stone of weight. We were travelling along at 18 knots myself on the helm and my companion on the deck when out of the blue the boat suddenly veered to port almost at 90 deg without the steering being moved I didn't know what was happening and nearly lost it,managed to throttle off and stop.The cause of this sudden change of direction seems to be the fact my friend moved suddenly from one side of the deck to the other,the incident left me well shaken I dread to think what would of happened if a boat was coming towards us.Came home with a box of Coddling but not a good day.have any other owners experienced this ? in future passengers will have to stay put when the boats on the move.T.J.