Most likely the kids were asked to leave because they were loitering i.e. hanging around DTD and not eating, drinking or spending in the shops. Possibly the kids were acting somewhat loud and disruptive, sitting around in a group that looked like a rowdy gang.
As for the brawlers, it's different from the other cases because it was a felony. Trespassing and loitering are misdemeanors because they are annoying but otherwise harmless. If someone commits a felony on your property, meaning they commit serious harm or endangerment to life and property, do you sit back and wait to see if it's a "pattern" ? Apparently Disney does, at least if the mayhem is guest-on-guest and not guest-on-employee or guest-on-Disney-property.