It will do that on a hot day. The fuel is stored under ground at the station so it's kind of cool. When it heats up in the cab of your truck, it expands and runs over. I had a Jeep that did that all the time. The filler neck was only a few inches long, so I would stuff the nozzle as far in as I could and not top it off unless I was going to use a few gallons right away. If you park the truck so the filler neck is on the downhill side, it exaggerates the situation.
