One thing the Brits (or at least the cops in London) learned a few years ago is that in the age of most people having a smartphone (or access to one) "planning something" is relatively simple, as long as you are not too picky about what you are planning.
Those London "riots" have kind of faded quickly from memory (the authorities really don't want people to remember how easy it was if you ask me) but the idea doesn't take much of an IQ to figure out.
You just pick a "Target" (bad pun sorry) and text all your "friends" and put it up on social media, twenty minutes to an hour later - boom!
I'm not saying I approve of the idea, just that it makes things a lot easier to suddenly mob, attack and regroup than it was in 1992.