Once labor begins in childbirth, contractions increase in intensity and frequency until delivery. Why are the increasing contractions an example of positive feedback?

First, a little bit about the difference between positive and negative feedback. Think of negative feedback like a thermostat. There is a specific set point that something needs to stay at (such as body temperature, blood carbon dioxide levels and blood sugar levels) and small adjustments are made either up or down to keep the body at that set point. 
Positive feedback is like a vicious cycle with a specific goal. Once it is started, it will continue and intensify until completion. Child birth is a good example, but if you are having trouble with that, think about the blood clotting response. When bleeding begins, the injured tissue releases chemicals that activate platelets to help create a clot. The responding platelets release chemicals that activate even more platelets, and the process continues and intensifies until a clot is formed and the bleeding stops.
In child birth, the hormone oxytocin is released when the uterus contracts, and the same oxytocin causes more contractions which releases more oxytocin. In this way the process continues and intensifies until the baby is born and contractions stop. Because the goal (the baby being born) was achieved by speeding up and intensifying the process, it is an example of positive feedback.