Lately I'm thinking economic collapse, perhaps started by one of those computer viruses that's currently running rampant in other parts of the world...hmmm...healthcare shutdown? WTF?
Economic collapse caused by a regular old biological virus, overwhelmed healthcare, people staying home so as not to catch said virus etc. etc.
Economic collapse following a tragic natural disaster. flood...earthquake...fire...who knows?
Economic collapse caused by poor economic policy over 4 decades...
I read a book several years ago where many virologists said the next big pandemic could be influenza. Something like the Spanish flu outbreak around 1917.