After his failure at the slave auctions, Krieger decided to cut his losses, and the number of mouths to feed, by pitting a third of his slaves (the PCs) against another third (some Raiders).
Prior to the battle, Earl conducted psychological warfare on Horsecock, convincing the raider to charge the pickup truck containing the slavers in an attempt to secure the collar triggers secured there. Meanwhile, Megan worked the growing crowd to convince them to bet on the PCs and thus lose Krieger some of his profit.
The battlefield was set up such that the PCs stood on one side of the shit field and the raiders on the other. Scattered on the ground between them were a number weapons, some more makeshift than others. Earl charged into the middle of the weapons, grabbed (then dropped) a parking meter, and proceeded to pummel raiders with his fists.
In the course of the battle, Horsecock did indeed attempt to charge the pickup truck and, as a result, lost his head. Shovelface, who wasn't known as such before then, took a sharpened spade to the eye but was miraculously not blinded. He did go out of the fight, though, and was one of the few raiders to survive Earl's onslaught.