Lol! My DS9 is EXACTLY the same way - my husband and I lovingly refer to him as Worst Case Scenario.... We've learned to placate his fears somewhat and try to keep him away from the news (hurricanes, earthquakes, natural and man-made disasters spur new fears). I attribute his fears to an over-active imagination.
As is my 11 year old. We also keep her away from the news. I've recently taken to positing equally wild scenarios when she starts going off the rails with the "what if...?" The wilder I make them, the more it distracts her and gets her laughing, and she starts to realize that maybe she doesn't need to worry so much about random things happening.
I think it's a completely normal part of being an adolescent. They're starting to be more aware of what goes on in the world and some of the things that can happen, and they're not as sheltered from it as they had been when they were younger.