A tandump can be performed either consensually or by ambush. In an ambush tandump, the assailant initiates a tandump by climbing over a stall wall and dropping down onto the unsuspecting pooper.
Jacob: Yeah of course! I call dibs on bottom!
Joe: Mike, why did you and Elaina break up?!?
Mike B: Her ex, Josh, told me that she really enjoyed tandumps, so I decided to surprise her with an ambush tandump! She threw up on me and threatened to call the police...