In reality, it is possible that they started the initial emergency descent with a simple spin down of the Alt selector and a pull to get it into open descent. With no other inputs the rate of descent of c3000fpm would be normal, although over high ground you'd normally select the msa
It seems the most logical explanation for starting a "normal" controlled descent as the available data suggests.
Incapacitation of the crew could’ve occurred quickly, as they apparently failed to navigate (at least away from high ground) and communicate (mayday if anything).