[Kenny Bunk]

Guardian reports:

1 dead,
1 "Serious" injury.

Cause:
"Serious anomaly." IOW, "don't know yet."

(Wild Ass Guess) Not that great a body of fatigue data on this composite stuff quite yet, and the thing has been flying with rocket power for well over a year..

[PIF]

I think this was a test of a new engine design. They had been down for over 9 months - this was to their return.