Overfilled Propane tank is what the firemen said...

If you have a round valve propane bottle, they would let the bleed leak liquid when the tank was full, letting the dispenser know to shut it off.

If you have a triangular valve propane bottle, then do not have a bleed, but they have an internal check valve to stop the flow.

If the filler had a round valve and didn't open the bleed, and gave you a metered amount, but there was already some in the tank, the tank would overfill. I reckon that is what happened.