Sorry gents, initial reports are often wrong.
Short of a million-to-one lightning strike causing a fire as reported, little else comes to my mind when a modern jet transport burns, then crashes rather than the other way round.
I may have been influenced by pictures of a certain someone in the news lately and the point of departure of that Ethiopian flight.
edit: initial reports, now amended, said that the aircraft was in flames as it went down. Now Sky News among others have changed their tunes.
I stand corrected until further information - accuracy improved - arrives.