Someone stabbed the leader mole with a knife to overthrow it. Now there is a new mole leader. Because that's what happens when you overthrow someone—someone else needs to take over. In this case, a vengeful mole.
Where do leaders get overthrown the most? In とうきょう. Just think back in Japanese history, back when everyone was at war with one another. People were taking over and getting overthrown and then that guy gets overthrown and then THAT guy gets overthrown. Basically, it used to be hard to stay a leader in とうきょう.