I'm thinking one of three possibilities:

1. They had installed a larger tank but hadn't changed the valves or whatever to handle that amount of weight so it gave out, and that same weight made most of it come out quicker than it would have done with the original tank.
2. Another custom change where they could empty the tank from inside the vehicle and while it was in motion.
3. The tank of that model (or if it was custom) was unshielded and ruptured from the weight, spilling out quicker than it could have done normally.

Either way, that's a shit load (hah!) of excrement to be released like that. I'm with you on wanting more details, someone must know.