When I heard about the accident, I immediately thought of the AJ-26 engine explosion on the test stand earlier this year. There was an investigation, an inspection of the entire engine stock, and a delay of the Orb-2 Antares flight. Eventually Orb-2 flew successfully in July.
Then I watched the Orb-3 launch video. You can see a brightening of the exhaust plume, followed by an explosion at the base of the rocket. Although there was a lot of fire, the rocket remained essentially intact until it hit the ground. That looks a lot like an engine explosion to me.
Obviously we'll need to await the results of the investigation to learn the root cause, but if it does turn out to be a problem with the AJ-26, that would raise the question of why it wasn't found during the earlier stand-down. (It has unfortunate echoes of the most recent two Taurus XL launches which failed in the same way. In those cases, it wasn't an explosion but rather a failure to separate the fairing.)