Yglesias connects the falling crime rate to positive feedback loops:
Consider, for example, the fact that there would be fewer murders in Philadelphia if murderers in Philadelphia had a higher chance of getting caught and sent to jail. But if there were fewer murderers in Philadelphia, then the cops could dedicate more resources to each murder and murderers would have a higher chance of getting caught and being sent to jail. So if the murder rate decline in Philadelphia, that decline has a good chance of becoming self-sustaining.
Serwer, meanwhile, eyes demographics:
As Kleiman explained to me last year, the most relevant statistic when it comes to crime is the number of people in a given country who are in prime "crime committing age" between 15 and 30 years old. That's why the so-called Baby Boom generation correlated with such a large increase in crime.
The Dish 