There is no aparent "thread" geographically. There is a sharp increase in the number of incidents from late 1999 to the Spring of 2001 though. An occurance during this timeframe happened almost once a month, if not twice. Now, I realize that this sounds quite omminous, but the school shootings have tappered off quite dramatically since conservatism has again gained popularity. This, I have noticed, is the only "thread" I can see.
The shock of 9-11 may have slowed it down for a few years after 2001.