84 300D Pressure in fuel tank

[hutsutraw]

So, this car is still new to me and I'm working things out slowly. I had something very odd happen to me the other day.

I was driving down the road (this car has a 2 tank veggie system) and pulled into the gas station. The night before, I had filled up the diesel tank to the brim. So, I walk inside the station, come back out and see something leaking all over the ground - it's diesel fuel. It's pouring out like crazy! I go around and pull the cap off the fuel tank and it just shoots diesel fuel from out of the tank all over me! Extremely under pressure and I don't know why. Has this happened to anyone else before or know what causes this?