If he goes into the forest with his neighbor to cut timber and swings his axe to chop down a tree, but the blade flies off the handle and strikes and kills his neighbor, he may flee to one of these cities to save his life.
It was one of the precepts given to the sons of Noah that whoso sheddeth man's blood by man shall his blood be shed, that is, by the avenger of blood, Gen 9:6. Now here we have the law settled between blood and blood, between the blood of the murdered and the blood of the murderer, and effectual provision made, I.
Commenting on Deuteronomy 19:1-13
As when a man goeth into the wood with his neighbour to hew wood,.... A wood is a place common to men, and cutting down wood a business which any man might do; whereas a private place, where a man had no right to be, and doing what he had no business with, rendered a case suspicious, and such a man was liable to be...
As when a man goeth into the wood with his neighbour to hew wood, and his hand fetcheth a stroke with the axe to cut down the tree, and the head slippeth from the helve, and lighteth upon his neighbour, that he die; he shall flee unto one of those cities, and live: (c) That murder not be committed on murder.