Hmmm.... That's a great question Den.
I'm not 100% sure but I have a few ideas.
I'd say that it's a lot o do with our old friend "cognitive bias", people see what they want to see. I also think people are "pre-disposed" to believe these strange notions. I think it stems all the way back to being a caveman, probably an instinctive fear originally used to protect us (the fight or flight response). Now that we don't have that type of predatory fear any longer - it now focuses on something else - evolution take it's time to fix things up.
A lot has to do with ones genetics also. Some people are programmed to have a more "pessimistic" outlook than others. This would also include why some people are more superstitious than others (although there are other factors involved too).
Paradolia also comes into play here I think. Again, we as humans start putting 2 & 2 together to get 5. What I mean by this is, we see a lot of disasters, then this becomes "a sign". A sign of the end of the world. Just like you see a few fake crop circles. Then all of a sudden it's a sign that aliens are visiting the planet, etc. You can [pretty much do this with any phenomenon. remember the virgin Mary depicted in a cheese sandwich.
After all it seems were "hard wired" to recognize pasterns.
I see a snowball effect happening here too. One person says "X", then it becomes a mild form of hysteria, as more people buy into it. It doesn't seem to be all that hard to convince people that everything is coming to an end real soon. Just like it's easy enough to convince people that Big Foot is about. Ironically most people believe in these things but have never had any actual experience with the phenomenon, whether it's ghosts or ETs. People, a lot of the time simply believe through hearsay, and anecdotal evidence. And you can ask any Cop just how unreliable they are.
Religion has a major part to play to, as it's been hammering away at us for years convincing us that the world will end some day - most of the major religions believe in this. I see it as just another form of control through fear. Same deal as the after life.... "you wont go to Heaven, if you don't do what we say...".
Just look at all of those killer cults. Nearly all have scared, and isolated their followers into believing that the world is doomed, and they are the only ones to be saved (some good logic that). Basically it creates an artificial fear, which gives us the idea - which then plays on our own natural fears as human beings (basically an artificial fear).
Let us not forget the media (and money of course). Disasters are much more likely to sell stuff to the average Joe. Just look at the newspapers and the "disaster movies" that come around every summer. These guys also have a habit of whipping people up into a frenzy, and getting them to be us congnicent of unnatural fears - even if they do it at a subliminal level.
That's about it for now.
If I can think of anything else I'll let you guys know. If anyone else has anything else to add, please do!
Cheers,
R.