A number of you guessed correctly the cause of the fire had to do with the smoke stack (or exhaust pipe) of the truck. The stack was too short causing the exhaust to blow right into the hay bales behind it.
Stray pieces of hay may also have blown down the stack while the truck was being loaded. While the truck was driving those pieces of hay may have become hot and blown right into the hay as well.
Both fuel tanks exploded as well as the tires of the truck. Hay bales also fell off the trailer as they burned starting small brush fires on both sides of the road. Thankfully the driver, who was the owner of the truck, was not injured. He thought the white smoke was just dust and didn’t realize his truck was on fire until the smoke turned black.
Credit for the great pictures goes to Bon Bon who took off with Mr. Ferrero Rocher to the scene of the accident when they heard the truck was on fire.