hi all
ive been watching a lot of drone fail vids, i can see some are just idiots who shouldnt even have access to a paper airplane,
but there were some where drone was just hovering and it suddenly flips or barrel rolls and crashes.
or its flying along at height and then becomes possessed and takes on a mind of its own.

what causes these glitches out of interest, i can understand poor battery etc but what other causes

Flips and rolls could be motor/props or ESC issue,  when they fly away that's usually a case of GPS glitch.