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It didn't start moving deliberately. The trailer has brakes, so when the driver of the semi tried to stop, it actually started pushing the barge.

This is why boats are moored to the land/dock. Motion onto a boat imparts motion into the boat. Walking onto (going from land onto a floating vessel) a boat makes the boat move in the same direction the person walking onto it is moving. For a person, the smallness imparts a very little amount of motion. For a massive truck carrying a load, it will impart motion that can actually launch the vessel.

There has to be a counter force to pull the boat back to land anytime something moves onto the boat. That's what mooring the boat does. The motion being imparted into the boat is then placed onto the lines holding the boat to land. If the motion is strong enough to break the lines, the boat will come free but if not, then the motion is imparted into the mooring posts. Depending on their construction, they'll either give way or disperse the motion into the ground.

If you've ever canoed, the boat is super small, so the sheer act of just getting into the canoe starts making it move in the direction you entered the canoe. Which is one of the reasons folks usually get everything into the canoe on land and then have someone launch the canoe from land.