Towards the end of the first season of Terminator: The Sarah Connor Chronicles, Sarah mentions a woman she knew when she was a waitress who had children she was trying to get into showbiz. The woman would say nasty things to the kids, like their teeth weren't strait enough, and pushed them hard to get into the industry. She says, to the chagrin of Derek Reese, "I can almost understand why they drop bombs on us," in reference to Skynet.
Well I think less than horrible parenting it would have to do with human behavior in airports and on airplanes. I swear, after a day in airports, I'm calling for the eradication of humanity.