The mayans have only predicted things that were likely to happen.
Many clever people can say 'Well, there's going to be another huge scandal about the world ending', and the mayans had clever predictions, such as 'We are not the only people on this world. It is a vast place, and we are probably not the only ones here. We are likely to be invaded by them', and things like 'If this happens, which it probably will, people are likely to be violent and we will have big wars.'
So they wrote things like that on the calander, and blargh.