Like many great things, this wonderfully crazy idea came from a little rule breaking. D-Van Grand Prix organizer Takuro Abe was at a track event for a motorcycle racing school, and vans were used to haul the bikes around. During lunch someone came up with the idea for a race. Ignoring that the big machines weren't actually allowed on the circuit, the drivers headed out. The popularity has just grown since then.
These days, the racing vans absolutely aren't the stock machines from the event's inspiration. In addition to stripped interiors and track rubber that you might expect, the list of mods for them is a mile long. For every possible advantage, the racers fit them with things like Brembo brakes, cross-drilled rotors, heavy-duty transmissions, and much more.
Seeing vans lumbering around the track is very weird at first, but the racers take the competition very seriously. These folks even employ all sorts of little tricks to coax the most from the machines. This is a fascinating motorsports story, but be sure to turn on the subtitles to understand the interviews with the competitors.