How do fad start and who starts them?

Typically, popular people. For example, if Selena Gomez and Justin Bieber tattoed a picture of each others faces on their foreheads, other people would follow. Why is this? Simply put, because they are so popular. Other people that start fads/trends:

-A popular cheerleader

-Chuck Norris

-Darth Vader

-Your Mom

-Charlie Sheen "winning" :3

and

-Me