I'd say there are a few things at work here. First, part of the problem with the statistics of small numbers is that a change of only one or two or three events is suddenly a doubling, or tripling or other massive increase in frequency. These sort of events do tend to be rare-- that's why they're still news-- but "random" doesn't mean that they'll be scattered evenly over time. Quite the opposite, they'll tend to clump up sometimes.
Second, the nationalization of news over the last couple of decades has made everything local. It used to be that when people watched the news for an hour, only a small part of that was national news. The local news would carry murders and this sort of thing, but only rarely would such a story rise to hit the national news. There wasn't bandwidth for it. But now, with 24 hours of programming to fill, it is easy for lots of otherwise "local" stories to become national news. It makes it seem like these things are happening all the time when in fact in any particular area they're of course still very very rare.
Third, all of the above notwithstanding, I was just reading a story (forget the source) this morning that said that sometimes these stories *do* help push the next one over the edge. The VT shooter was said to have been inspired by Columbine, for example. It's not a stretch to think that some borderline nutcase will see one of these stories on their TV and think "Hey, I'm that mad too!" ...and maybe it gets them thinking in a very dark direction.
So... there's my .02
Bump!