I don't know why this happened (or if the truck somehow stumbled upon some secret Mario Kart item that lets it shoot out fiery smoke rings like a red shell) but it's such a cool effect. The smoke that blows out of the truck's engine forms perfect o rings and then bursts into flames.
Think you're cool cause you can blow o's when you smoke? Mother Nature has still got you beat. Mount Etna in Sicily, Italy took a little break from erupting to blow O-shaped smoke rings hundreds of feet in diameter to make every smoker and pot head in the world jealous. Just look at it. It's perfect!
Mount Etna is now erupting in Italy, spewing hot lava everywhere. Back in 2000, Etna was much calmer. She spent her days chilling out, blowing pretty smoke rings, which is apparently is a extremely rare occurrence for volcanoes.