Violence has consistently gone down over the past few decades. Yeah, the homicide rate climbed in the 1970s and 1980s and peaked in 1991, but it's been in freefall since. It can't really be correlated with secularism, though, as that's been the norm in American society and politics. If anything, you can correlate the spike in violence with the rise of fundamentalist religiosity in the post-Counterculture era.
Most likely, though, a lot of it is just that violence is being more consistently and more thoroughly reported and scrutinised than it has ever been before.