It was hard for me to find again, so i'll leave this here if anyone else is interested. The psychology behind a lot of how rumors form and perpetuate fall under herd behavior, availability cascades and information cascades. Pretty much all of it is based on a group of people deciding that something they all agree on is important, such as a significant threat, or a particularly good outcome. Everything from public policy to stock market prices to internet memes and media scandals seem to be decided this way.

http://en.wikipedia.org/wiki/Herd_behavior http://en.wikipedia.org/wiki/Availability_cascade http://en.wikipedia.org/wiki/Information_cascade