I notice from Sandra's reply above yours, that he was probably a mechanic going by the Mech part of his rank.
I wonder if something went wrong when he was repairing a plane, maybe there was an explosion of some sort.
It may be worth getting his death certificate as that may tell you what he died of.
APTED, WILLIAM 39
GRO Reference: 1916 D Quarter in CRANBROOK Volume 02A Page 1218
Cranbrook covers Hawkhurst. I know his age appears to be slightly out according to other records, but it is only by one year.
You can get this death as a pdf which would cost £7.