Not Necessarily the Engine
Everyone online seems to be saying engine at this point, but so far as I can tell from 3rd party photos it could have been airframe failure, or any of a number of things. I don't see evidence of an explosion (frequently cited), just escaping nitrous oxide. Just because it was a rocket test doesn't make it a rocket failure.
(And yes, I have professional experience in precisely this area, though no specific knowledge of this vehicle.)