Most two common things IMO, which seem realistic to me, are:
1.) Asteroid impact, darn things are flyin near us, and they don't spot every one, and by the time they do spot one that is headed toward Earth, it is usually too late to do anything about it. There are near misses, and I think there is a possibility of a hit eventually, I know there is at least one Asteroid that is scheduled to come close, Apophis I believe, I know its a ways out, but what if there is another that we just haven't seen yet.
2.) It is fact that 2012 is a year of Solar Maximum, meaning, increased solar activity including solar flares. Perhaps these flares will impact our satellites and other electronic equipment on Earth, not saying it is end of the world type stuff, but any disruption in our electronics and communication can affect a lot of people, and could cause panic if there is widespread outages.
2012 could be what they had feared for Y2K happening, if there was say an EMP from the Sun that knocked everything out.
