Back in the days Matt Serra was an badass. You can find serveral videos of Matt Serra fighting guys in unplanned situation. Even in the latest episode of ,,Dana White looking for a fight" he started a grappling match against Nick 'The Tooth" In this flashback video you will see Matt Serra in a grappling match against a big random guy. All the other guys are going crazy to watch that..until the police steps in.
Why were police involved in that? They were fighting by agreed upon rules within a competition environment.