The event could play out alot like the zombie siege, and even re-use some of the same mechanics.
• Start off with a message "Hey, somebody showed up at the front door, and they don't look happy. Could you talk to them and see what they want?"
• If player is not home, start a 10min timer (like the zombie siege), and if player doesn't return in time, subtract some ammo, and one of your survivors says "They started shooting, so we shot back. They're gone for now." (Maybe even injure a randomly selected survivor?)
• If player is present to talk to them, they make demands "give us a [rucksack, generator, etc.], and we'll leave in peace."
• If player refuses, they turn aggressive and try to kill your survivors.
This could be limited to the dread zones if too overwhelming for standard. Or the event could be tweaked for the higher difficulties (ie. more waves of hostile survivors spawn after you kill the first ones).
It could be explained that these groups are connected to the same antagonists in the Sherriff questline, or simply enclaves that you ignored for too long. Neglected enclaves usually leave the map, or turn hostile, and this could be the next step in that process. Instead of sitting in one spot, being forever angry, they take a more active approach and try to make you regret not helping them.
Ps. Thanks to everyone at Undead Labs for making this game!