If by boiling you mean bubbling it probably has to do with the introduction of greater current in the water due to the physical movement of the pot and therefore a greater frequency of pressure pockets where hot enough water separates into water vapor, but more bubbles don't necessarily mean hotter water, and the same effect could be achieved by dragging a cooking spoon through the water around the same time, and I believe to have also witnessed it by pouring salt into my pasta pot just as the water was about to boil. In neither of these scenarios is the thermal energy increased in a measurable way.
