Check the SimCity 'Disasters' menu. Earthquake, tornado, meteor strike, plague, zombie attack, not-Godzilla...

Also civil unrest. Riots, revolutions.

For bonus points: a bunch of adventurers rolling into town. Make them look uncannily like your current party. Show them drinking in a tavern and discussing some big job, without ever actually saying what it is. Try to imply that this was (very) shortly before a major, also-unspecified catastrophe struck the city.