If you destroy too much of a town/city or a event (like zombies, radiation, mass prison breaks, ooze outbreak, etc.) occur;
A state of emergency could happen with sirens and what not. Forcing the people to evacuate. Most likely leaving their belonging and establishments unprotected (well the guards will be gone just watch out for traps). of course some people may stay and even invite other people in.
Alternatively a location could be quarantined/locked down with police guarding the main roads and It will be illegal to enter or exit the town/city for a time. forcing you to go through other routes to get out of that place If you can't risk a fight against dozens of trigger happy police waiting for something to happen.