Similar to the "Allspark" in the Transformers franchise which sparks independent life in electronic/mechanical objects upon contact, the Tardspark is a person who acts like a tard and sparks a spontaneous chain-reaction of tard-like behavior in those people around him/her. A person who encourages and/or instigates tard-like behavior in others.
Otherwise peaceful gathering in the movie theater courtyard. Suddenly, tard A starts yelling at and mooning passersby - convinces his friends to do the same. Security guard shows and arrests all. Onlooker shakes head and comments, "the fucking tardspark strikes again."