That is the aspect which raises questions for me - though they are trained to be assertive even when someone is appearing to be complying and it isn't clear what interaction that second guy may or may not have had previously with the police. But not even restraining the first guy, and that doesn't seem to be part of a process i.e. first officer neutralise, second officer cuffs or something, and then jumping on to the second guy in that manner appears both unprofessional and poor practise potentially putting his colleagues in danger.
But so much missing or vague information at this point.
Reminds me of a bouncer in a local nightclub - little jumped up twerp who wanted any excuse to brutalise people without them being able to fight back - eventually ended up badly for him.