I would imagine pack mentality had kicked in by then. I take your point though. This wasn't a 'one punch' assault that resulted in a death (not that that would be acceptable either). It was a prolonged assault that at any point any one of them could have either stopped himself or called on the others to stop. Poor training? A police culture that encourages officers to see any potential arrestee as not worthy of decent treatment? If the full footage is as damning as this looks, I think the defence will run along 'heat of the moment' and blame shifting 'that's how we were taught to do it' lines.
From footage I have seen, which is both from the officers' recorders, as well as the CCTV, (not the complete footage from both sources), it is an appalling sustained attack.
Two deputies have also been suspended and I would imagine that further charges will follow to those already sacked, with offences in addition to the half dozen or so with which they have already been charged.
I am sure their training manuals cover how the Tennessee police should react to the different scenarios they are likely to encounter with motorists, but generally speaking I would think American police dragging someone from a car after it has first stopped was a less common means of apprehending the driver. Certainly from videos I have seen of patrol car officers conducting traffic stops, even ones involving the vehicle being chased, the primary concern of the officer is being shot from the stopped vehicle. So the approach tends to follow a safer pattern rather than a headlong charge to the driver's door.