Accidents are very rare, negligence is more common. No one here was there, so no one here can say whether or not the firearm failed or he was being negligent. A firearm is a man made machine, we are not perfect, we do not make perfect machines, they can fail.
It would be nice, if the media portrayed the firearm with respect, and stated the firearm failed and a round was discharged unintentionally. Until the investigation (if there is one, but that's a different problem) comes out, who knows what happened.
It would be nice if they stated where the bullet went. Ie: if it struck just below his chair, where he was sitting, and his holstered firearm was directly above the hole, maybe it was a mechanical failure. Or, if the hole was 5 feet away on his non weapon side, it was most likely in his hand.
It would be nice to know if the firearm cycled properly. If the firearm was in the holster, there is a good chance of a cycling failure there too.