A thin layer of air near the surface became extremely hot and unstable this day (the surface temperature was 108F). However, the layer of moist, unstable air was too thin to build a thick cumulus cloud. When the unstable layer began to rise, it could do so only as an isolated bubble of air. This rose quickly and hit a very stable layer of air a few thousand feet above, where it spread laterally.

This looks like a mushroom cloud because the physics is exactly like that involved in a real mushroom cloud.