Here in FL I've seen ones similar to that a couple of times.
In fact, I've seen one that was even more spectacular because the top layer of it was thin and wispy, and had spread out in a PERFECT circle, covering a huge section of the sky, and the circle had spokes in a regular pattern in it. Most amazing thunderstorm cloud I've ever seen.
Things like this happen when a thunderstorm grows very high and when it gets to its peak heights, conditions don't allow any further upward cloud movement. It's a ceiling to further upward development, and it starts spreading outwards instead. Normally, the upper level has a prevailing wind direction, so the top of the cloud shears off in a particular direction, but in cases like the one pictured, that top layer is calm air so it spreads out in ALL directions instead.
CJ