That is so cool!
Agreed. Due to a human phenomena known as pareidolia
http://en.wikipedia.org/wiki/Pareidolia there is a lot to be seen in the cloud structure. I see a penguin, sphinx, and a horse's mouth with teeth. Also an UFO?
Also, doubt the pileus cloud actually disappeared, but was no longer visible as the sun angle dropped down as the sun set, and the refracted light no longer could be seen, as in a rainbow, so it is transient in nature and only seen under the right circumstances. Could be much more common because of that. Whatever it is, there is a lot of energy visibly being displayed as the cloud is morphing into a thunderhead.
It's a good site.