He, the God who leads you, will expel the nations little by little. You will not be allowed to destroy them all at once lest the wild animals overrun you.

When the Lord your God brings you to the land that you are going to occupy and forces out many nations before you – Hittites, Girgashites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites, seven nations more numerous and powerful than you –

Suppose he goes with someone else to the forest to cut wood and when he raises the ax to cut the tree, the ax head flies loose from the handle and strikes his fellow worker so hard that he dies. The person responsible may then flee to one of these cities to save himself.