I guess the deal was he felt like his vocals were drowned in the mix. In between the songs Farrell was talking to the audience and Navarro belted out a power chord to shut him up. In the song they were playing before the scuffle, apparently members of the audience in front were shouting they couldn’t hear the vocals. Farrell “screaming” at the end of the song was him trying to highlight that even if he was screaming no one could hear him, and I guess that amped him up and then he snapped.

In my opinion, basically what happened is he was drunk and babbling and slowing down the show between songs. Additionally, I wouldn’t be surprised if his vocals were lowered in the mix given he admitted himself after a previous show that he basically wasn’t singing well. Not uncommon to have the rest of the band a little louder to cover for a struggling vocalist.

Simply put, he was drunk and not singing well. But mostly he was drunk and being a babbly asshole, and he was drunk and turned violent.

Basically, he’s a drunk asshole.