Lawless and collapsed government are not quite the same thing. Government also operates on several levels (municipal, state/provincial, and national)...I'd assume you are going to want something that drops all three levels.
Viva La Revolucion! First method of dropping a government to its knees is the people themselves. Let's say the powers that be behind the Anonymous movement get what they want (anarchy)...you'd see coordinated anarchy on all levels (the idea here is that lack of government does not equal lawlessness). Not so sure if this will fit the bill for what you want though, committed anarchists believe in no government and not committed to lawlessness.
I'm assuming you are looking for more of a disaster that makes the government incapable of functioning. For this to work, you need something to affect the populace heavy enough that there is no longer enough people to maintain the current government (which is actually a pretty large number of people, the bureaucracy will expand to meet the needs of the expanding bureaucracy after all).
Plague / sickness is my first choice here...a government cannot operate if the employees of the government are too sick (or dead) to keep it functioning.
Radiation / nuclear war. If you want to empty major cities fast, put the threat of a nuclear strike on it. A global event that causes a catastrophe that damages nuclear reactors and causes them to meltdown worldwide (Fukushima for all!). Even a super villain that managed to detonate the majority of nuclear arms in their silos would work here.
Entirely irradiating a nations food source would probably work to the same extent, cities need large amount of food to survive, a mass shortage of food can empty them and reduce it to lawlessness as we compete for the remaining food supply. Even a worldwide food shortage/disaster could have this effect.
Neutron war. Maybe I'm off in this theory, but there is a 'neutron bomb' idea that has a bomb that is relatively 'weak' when it comes to causing damage to buildings, but absolutely devastating to the population in those buildings. This would be a far more instant version of a ghost town. Drop enough of them and you'll have your anarchy.
The options above allow for a ghost city where the city is relatively unharmed (radiated maybe?) but the population has mostly disappeared. I've got some other options, but they don't really leave cities behind.