Microwaves do not 'bounce around' in the oven. Microwaves are
directional, that's why you move the food. The microwave emitter
does not move.

On some cookers, that isn't strictly true.

To quote Galileo: "Nevertheless, it does move." Once upon a time, microwave
ovens had a "stirrer", a fan that "moved" the standing-wave pattern around
the oven cavity.