On June 27th, 2012, Valve released its final Meet the Team video, Meet the Pyro
This video contained a particular scene in which the Pyro was imagining himself in a fantasy land where he was playing with Cupidesque imitations of the BLU team when in reality he was viciously slaughtering the aforementioned team.
Before long, other users started making parodies in which the Pyro has a different hallucination, some of the earlier users doing so being Supermariogalaxy13 and Mistermild
http://youtu.be/BXelSyTtAwk
The former made a template that can be used with greenscreen editing to make their own version, so others starting popping up, creating a fad
Some examples