"Prophecies" about 2012?

Firstly, I do *NOT* believe this fear mongering hype. I am, however, curious as to the coincidence that different cultures and people that had no contact with each other prophesied that same date - 2012. These claimed sources include Nostradmus, the Freemasons, I Chang, the Mayans, and a few others I believe. Is there some kind of explanation to this? Did these groups and cultures in fact not predict the world to end in 2012 but rather theorists speculate they did because of the hype?