While researching something unrelated, I came across this (PDF) odd chart. It's a clear trend, but a totally puzzling one.
Also a trend:
It could make sense if you buy the theory that murders in Chicago are increasingly the territory of gang members (as opposed to domestic or other conflict-related); for obvious reasons, gang members would be more likely to encounter one another in the public way as opposed to a residence. Otherwise, I'm baffled.