It's easy to invent scenarios. But the high probability estimate really derives from two things.
First, the special date from the Mayan calendar is astronomically determined, to a degree that hasn't been recognized by mainstream scholarship about Mayan culture. The precession of the equinoxes takes 26000 years. Every 6000 years or so, you have a period in which a solstice sun or an equinox sun lines up close to the galactic center, as seen from Earth. We are in such a period right now; I think the point of closest approach was in 1998. Then, if you mark time by transits of Venus (Venus was important in Mayan culture, being identified with their version of the Aztecs' Quetzalcoatl), that picks out the years 2004 and 2012. It's the December solstice which is the "galactic solstice" at this time, and 21 December 2012 will be the first December solstice after the last transit of Venus during the current period of alignment.
OK, so one might suppose that a medieval human civilization with highly developed naked-eye astronomy might see all that coming and attach a quasi-astrological significance to it. What's always bugged me is that this period in time, whose like comes around only every 6000 years, is historically so close to the dramatic technological developments of the present day.
Carl Sagan wrote a novel (Contact) in which, when humans speak to the ultra-advanced aliens, they discover that the aliens also struggle with impossible messages from beyond, because there are glyphs and messages encoded in the digits of pi. If you were setting up a universe in such a way that you wanted creatures to go through a singularity, and yet know that the universe they had now mastered was just a second-tier reality, one way to do it would certainly be to have that singularity occur simultaneously with some rare, predetermined astronomical configuration.
Nothing as dramatic as a singularity is happening yet in 2012, but it's not every day that a human probe first reaches interstellar space, the black hole at the center of the galaxy visibly lights up, and we begin to measure the properties of the fundamental field that produces mass, all of this happening within a year of an ancient, astronomically timed prophecy of world-change. It sounds like an unrealistic science-fiction plot. So perhaps one should give consideration to models which treat this as more than a coincidence.
Why pick out those events?
It's easy to see it as a coincidence when you take into account all the events that you might have counted as significant if they'd happened at the right time. How about the discovery of general relativity, the cosmic microwave background, neutrinos, the Sputnik launch, various supernovae, the Tunguska impact, etc etc?
I was very interested in the discussions and opinions that grew out of the last time this was played, but find digging through 800+ comments for a new game to start on the same thread annoying. I also don't want this game ruined by a potential sock puppet (whom ever it may be). So here's a non-sockpuppetiered Irrationality Game, if there's still interest. If there isn't, downvote to oblivion!
The original rules:
Enjoy!