Not at all.
Happens almost every year.
Has to do with the way they run refineries.
For the summer vacation/driving season, people buy lots of gas, not so much heating oil. Diesel fuel is produced along with the production of heating oil, not gasoline, so the market sells gas and demand drives up the prices every summer, while heating oil languishes at the distributor and prices fall. In the winter, everybody buys more heating oil, not so much gas, so heating oil (and diesel) prices rise and gas falls.
I've been watching this phenomenon now for over a decade. And if you look up historical fuel prices, you will see this goes on every year. Some years more so than other, depends on how mild the winter was.