now lets be real, the only reason this empire was forgotten was the fact that no one could read the written language.
As for theories, I believe that it had to be a civil war.
Reasons...
1. There would be records of conquest if an outside force took over the great nation of Nubia.
2. Nubia had a lot of gold, meaning they had a lot of allies. Thus, more would of been known about the demise of Nubia. If it was invaded I mean...
3. If they influence the Egyptians so much, chances are, they used the same type of work method. Meaning, slave labor. Not to mention, the fact that they had trade agreements with the Romans. Treaty with Rome means new slaves coming in. Meaning, slaves that were probably former royalty in their own right. Which would mean they would resent the Nubian way of life. Only under privilege part of a society would destroy holy relics in this way.
This is my theory, and it can be corrected.