Apparently, prices on the Turnpike had been a bargain over the past week, and prices at gas stations on the Turnpike had merely caught up with local stations.
A Bargain No More: Gas Prices Leap 17 cents on NJ Turnpike, GSP Parkway
They only update prices once a week.
When prices are rising, a lot of drivers will make their way onto the New Jersey Turnpike or Garden State Parkway just to take advantage of the lower prices from the previous week. Conversely, when prices fall quickly there is no reason to buy gas on these roads if you have an alternative.