After glancing over the pictures and viewing the video you should now have a better understanding of what the Planking fad is. Why people Plank, however, is a question that will remain a mystery to people who think rationally.
The origins of Planking are widely disputed, with various assholes trying to claim the responsibility for bringing something into the world that can only be rivaled by Justin Beiber's parents in terms of sheer uselessness.
Christian Langdon and Gary Clarkson are also often credited with inventing Planking. They claim to have invented a game called the "laying down game" as young teenagers out of boredom. What is impressive is not that they may have invented Planking, but that they may be the only teenage boys in the world whose free time was apparently not spent playing video games or masturbating.
Another possibility for the origins of Planking can be found on any college campus. I know that I often involuntarily planked after becoming intoxicated at college parties or bars. I was even more "extreme" than modern day plankers because I added puke piles and pissing in my pants to my planks.
Love it or Hate it, Owling and Planking are popular fads that will hopefully go away but may torture us by evolving into something even worse. As to why people Plank, it will continue to be one of society's greatest mysteries. As long as there are idiots in the world, there will be always be people who do stupid shit.