Check out this accidental discovery, note how the graphs remain nearly parallel over years (!) http://www.wolframalpha.com/input/?i=violent+crime+in+seattl... Weird coincidence?
Here's something else: http://www.wolframalpha.com/input/?i=violent+crime+in+washin...
http://www.wolframalpha.com/input/?i=violent+crime+in+washin...
Hypothesis: the country at large has a crime rate. In addition there is a magic variable which only affects cities, but affects all cities:
http://www.wolframalpha.com/input/?i=violent+crime+in+san+fr...