Do you think the security guards just got an idea to tackle someone, on their own?

Someone upstairs probably thought they saw something and told them to hold the guy. That doesn't mean they didn't get the wrong guy or something else that went wrong.

But before you blame the security guards, and ask for them to be fired, consider who tells them what to do from the eye in the sky vantage points.

If they acted on their own, I agree with you.