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.