While held in over 60 cities across the world, this particular act of pants-lessness is spear headed by a local group called Improv Everywhere which is a prank collective with no particular agenda aside from generally spreading joy and amusement.
Below you can see some of my personal favorite scenes Improv Everywhere has caused throughout the years. Think flash mobs, but better. I am continuously impressed by the amount of effort they are willing to go through to fulfill a comedic task only for the amusement of others. Their missions are cleaver and often simple enough you could walk by them without ever knowing you're the butt of the joke. Their tagline "We Cause Scenes" is the perfect summarization of what they do best.
If you'd like more information (including getting on their mailing list if you want to actually participate in these events) you can find them online at http://improveverywhere.com!