There is some footage missing, which does raise a red flag, but what we see here doesn't seem justify the cops even getting involved.
Random fans of the opposing team are strongly defending the guy. If he was truly antagonizing them or trying to pick a fight, they most likely would've applauded his arrest. This seems to point to him being a loud, expressive fan, but not a trouble maker. I've sat next to fans of the opposing team who were quite passionate, but I've always been able to get along with them. The last Cubs v Cards game I was at I was next to a whole row of Cardinal fans and we all had a blast talking baseball and cheering against each other.
Also, public drunkness is a BS charge. If that's the only reason they had to arrest him, then why was he singled out when probably a third or more of the stadium was guilty of the same thing?
Someone posted something about being pre-emptive. Law enforcement isn't allowed to be pre-emptive. You can't arrest someone for a crime you think they might commit.