All cops are human, and humans tend to have terrible memories, lie regularly, are morons, and put self-interest over everything else.
The issue is that humans aren't held accountable for what they do and say in proportion to the amount of power they wield.
To be honest, I'm guessing that they're trained to push their body weight into the person's back, but only trained in a controlled environment with compliant trainers, rather than someone out in the open with others yelling, screaming, and throwing objects while the subject is trying to resist.
Ten guys try to do it with minimal training in terrible situations, and at least one of them is going to fuck it up like this. While on camera since everybody's got one in their pocket nowadays.