There's two things at work here.

First of all, you're brain is wired to continually process lots of information at once - and you do this unconciously. Now, if the information coming is is threatening to you - even if indirectly - you're unable to do anything other than react to it. Thus, when you watch the news and some disaster befalls some people far from you, you can't help but consider what could happen to you - is it a real threat? Your concious mind can tell you in a split second what's a threat and what's not - but your unconcious mind just reacts to what it sees - no language needed.

The other thing that is happening (and I admit it's happening to me to) is that you're seeking out patterns in randomness. This is the same thing that lets you imagine that clouds are in the shape of animals, faces, what have you, and it's also the same thing that lets you come up with a solution to a problem you may be having - those "a ha" moments that come along now and then. So, you hear threats of weather disasters, terrorist attacks, environmental catastophe, and that's in there while you seek to detect a pattern that will give you some better intel on the situation - some indicator, a predictor that will give you the extra seconds, minutes, hours or days to deal with the situtation.
I go through this frequently with fire calls. We sometimes go a long time with no calls at all, and then we get clusters of calls, and I always try to figure out what the indicators are - full moon? social security checks? height of the tides?
There are no patterns, and that's what's maddening. Yeah, we get brush fire calls in the spring, and car wrecks in the winter, and marine rescues in the summer...but I can't say "Oh, the moon is full and it's near the middle of the month, here come a bunch of calls"

Anyway, it's tough to be on edge all the time. It's better to admit stuff is random and leave it at that then spending all your energy worrying about what if and when could and what might. Deal with the real, prepare for general situations and don't worry.