Last call on departure... no more R/T (known!) exchanges until the crash... no apparent good reason to make a good plane crash other than wx...
.. Or someone smuggling on a 20 lb propane bottle .. that then started leaking. This is Africa, remember - where what helldog suggested (illegal cargo via corruption), would be an all-too-common occurrence. Can't see where wx would make a perfectly good, near new plane, fall out of the sky without a single R/T re problems. Catastrophic failure caused by on-board explosion, or poor maintenance, must rate highly in this case. My condolences to all those who have lost loved ones and friends.