Long before the war, a group of ex-miners including Megatron, Frenzy, and Rumble took part in a violent uprising against leaders who had exploited them while pocketing the fruits of their labor. They found themselves on the lam in crime-ridden Kaon, which is where they apparently met Clench. Clench employed these same bitter ex-miners to work as gladiators in an underground bloodsport (in which he also occasionally partook), placing them at greater risk than they had ever known and conditioning them to give in to their murderous impulses, all while hoarding the fruits of their labor for himself. Clearly, he had thought this through.