Yes, looking at the twitter feed, multiple sources state that it blew up. This means that either the tank (which takes up the whole rear half of the vehicle) failed catastrophically (unlikely; CF-wrapped tanks tend to develop hairline cracks which slowly leak out) OR the engine exploded.

What's interesting is that the SS2 engine fuel was swapped out very recently for a new thermoplastic polyamide solid fuel for better performace. Maybe this new fuel was not tested as well as it should have been?

static wick failure, minor fuel leak, big bang.

That said, I really have no clue about how these things work. That's just one scenario I could see leading to a midair explosion.

An oxidizer leak isn't going to explode.

(And it's pretty hard for a leak of anything to explode much when there's nowhere for it to build up. Limited internal space, wind rushing by...)

My bet is a hard start.