Have a look at the photo's,a very nice example of an Isolated Cumulonimbus Incus (very much like a Nuclear Bomb explosion cloud)
View photos an videos of it here
Based on what I have seen, a strong cap must have been in place, have a look at the base, usually there would be Tcu around.
When the Cap is quite strong and becomes breached, such in this case, can make updrafts and convection quite explosive and enhance the storm and instability as a result.
Very much like shacking up a fresh fizzy drink bottle and then opening the cap
These situations when there is a isolated storm with no scrappy mid or low cloud I find the best and it makes the storm much more impressive
It apparently, 'covered several miles and stayed in the air for about an hour, gradually taking the shape of an atomic bomb explosion. It then gave way to a further spectacle of lightning strikes, which lit up the evening skies'.