Some sources and scholars maintain that the art of mummification as practiced by the Olmec and Inca peoples was imported by travelers and refugees from Africa.
They came before Columbus, by Ivan Van Sertima, is revealing in this context.
ANCIENT ARTIFACS
http://www.youtube.com/watch?v=EeLRpFJ8gfcAs to your first and original question...
...Why did the people of Kemet mummify their dead or at least some of their dead?
I do not know.