One already venerable theory of why crime rates fell in the 1990s: ROE v. WADE. The theory holds that unwanted children are more likely to become criminals (18 years or so after their unwanted births) than wanted children, and fewer unwanted children were born in the years subsequent to 1973 than in the years before it. This, rates of crime began to decline when they 'should' have.
Here is more information on the theory and the evidence invoked to support it.
I've recently encountered another view, though. Perhaps the decline was a matter of leaded versus unleaded gasoline. .
In the magazine "Mother Jones" in 2013, Kevin Drum made the point that tying the crime rate to lead emissions has the economical virtue of explaining both the rise and the fall. "lead emissions from tailpipes rose steadily from the early '40s through the early '70s," and violent crime rates rose along a dramatically similar curve with a 20 year lag.
ROE v. WADE, on this theory, is a red herring, an opinion that just happens to coincide in date with the moment when unleaded began to replace leaded gasoline.
You can go here for more information on that one.