One speculation I have is that when the cop grabbed his shirt, it pulled up, and he saw a weapon stuck in the guy’s pants. Maybe a gun, maybe a knife.
Although if either of these were true, I would have through that we would have heard about it within an hour. It is extremely important in these situations to get the truth out before the governor can lie about it.
https://www.foxnews.com/us/kenosha-protest-police-officer-assaulted-brick
Scroll down in that story to see what the WI Governor said. With that and the likely politics of Kenosha, I doubt that anyone will be allowed to release any information early to exonerate the police involved. These matters instantly go to the top of the Democrat Party for tactical and propaganda considerations.