It is difficult to see exactly what happened in that video but my guess would be a negligent discharge. There are a variety of reasons or causes for it but it could have been when the officer was re-holstering or just simply poor trigger discipline. I also read that there is speculation that it was confusion over Taser versus pistol. Who knows if that was the case here but it has happened before but that was when the Taser M19 was being used and that thing was shaped like a large Glock. The newer Tasers are the X26 and they are specifically not ergonomically or visually similar to any firearm.
It is unfortunate that these terrible incidents turn into an excuse to act like maniacs. Yes, looting and destroying other peoples' property sure makes a statement about police brutality.