it is not understandable that a trained police office would react like that. That is why we have "trained" police and not groups of ad hoc militia protecting us.
These officers need much more training. All of them do if this is how they are going to react under these situations.
What happened before was completely out of order and needs the full force of the law and the justice system thrown at it, and i hope the injured officers are ok and getting the support they need.
But any officer who loses their head like that and stamps on someone who is prone, on the ground, not moving? Needs disciplinary action and training. and possibly therapy.