Hi Rory,
It was an author making an idle speculation, then a series of crackpots.
Author Michael Coe, in a 1967 book about the Maya speculated that, if the Maya thought this world was created at the beginning of this cycle, maybe they thought it would end at the end of the cycle. Then a screwball named Jose Arguelles wrote about it and claimed that the end of this cycle was the end of the calendar. He also claims to be a reincarnated Mayan priest, changed his name to Valum Voltan, and tried to sue the UN as "enemies of the biosphere" for refusing to change to the 13 month Mayan calendar.
Another crackpot, Nancy Lieder, first said the world would end in 1995. When that didn't happen, she changed it to 2003. When that one also failed to occur, she needed a new date, so she chose Arguelles' claimed end of the Mayan long count calendar, and predicted that the world would end in 2012.
There have been a number of others who jumped on the 2012 bandwagon (or moneywagon). You can read about them on their respective pages listed under Proponents on the left side of this page.
The Hysteria Channel and the internet have made this EOTW bigger than all the previous predictions.
