I mean does that sound like somthing you think happened?
Could it have happened? Yeah, sure. I mean, it used to be downright commonplace. The local Klan guys perceive some darkies getting uppity and go get their buddies and wreck ****.
You've got some kind of mental block about spontaneous group violence, but it's not only entirely possible, it's actually pretty likely. You just miss the history of peaceful protest going violent, or the fact a lot of US history has occasional instances of people getting liquored up and shooting up their neighbor's houses for no good reason?
Ain't like groups
add restraint when it comes to getting outraged. Usually the opposite.
Interesting how most incidents seems to congregate around the warmer months, as well as on the weekends. 
You honestly seem surprised that criminals are like the rest of us when it comes to when they want to stay indoors. They're not aliens, man. Superbowl Sunday is literally the quietest day of the year in terms of crime, too. (Valentine's Day is the worst, in terms of SWAT callouts.)