I was watching this amusing video and she mentions a couple minutes into it a
"Lazarus relex". Someone who is brain dead, but the doctors are keeping their
body alive for organ donations, may make movements. She demonstrated the
movement of her arms - crossing them over her chest just as how the pharaoh's
mummies were posed.
So that must be where the pose originated, I
imagine that the ancient Egyptian embalmers came across that pose often enough.

Anyway, the video is "Mary Roach: 10 things you didn't know about orgasm" -
about a third of the way in to the video:
http://tinyurl.com/q7nomz

I googled "Lazarus reflex" and found only a little bit more:

Here's a reference from a Catholic site to the Lazarus reflex. Often it involves
the corpse sitting up, and the corspe may or may not cross his/her arms:
http://forums.catholic.com/showthread.php?t=50919
"Dr. Conrado Estol, a neurologist from Argentina, spoke strongly in favor of
harvesting human organs. He presented a dramatic video of a man diagnosed as
`brain dead' who attempted to sit up and cross his arms (an act Estol called a
`Lazarus reflex.')"

I found an interesting reference here, from a book titled "The Facts of Death".
The author says that often the reflex happens when the doctors make an abdominal
incision to remove organs. This is very,very interesting because ancient
Egyptian embalmers also made abdominal incisions to remove organs.
http://tinyurl.com/pg4zu7

- Tamara