Why did a division of labor occur when people started to produce a surplus of food?

A division of labor occurred when people started producing a surplus of food because it was no longer necessary to use all of the labor force to subsist off of agricultural goods. Once a society or community can produce enough to survive and eat the labor force will begin to work in other industries creating a division of labor this could include making textiles, building homes, or any other number of economic activities. 

Internal combustion engines