I think the root cause is not a moral code or karmic debt or anything similar. One thing is to make the population more receptive to an idea. It's a kind of public education, they tell you that these things are a possibility and how should you react to them. So when the event happens people will start to behave accordingly what they "learnt" from the tv/movies etc.

The other reason could be related to the power of the mind. We know how powerful can be the placebo effect for example, if you truly believe that a sugar pill will heal you, there is a significant chance that the "therapy" will work. If you believe the same empty pill will harm you, it probably will according to the nocebo effect. If you disseminate an idea into the collective consciousness, and enough people will start beliveing in it, could that mean that the chance of the event happening increases? I think so.