Grace is right about the fuel vapor. Read a safety bulletin that explained it perfectly. The hot metal doesn't even need an open flame. It just has to be hot enough to ignite those fumes. That cool-down period lets everything settle. It's a simple step that stops a major risk. Makes you wonder why it wasn't a rule everywhere from the start.
You let them fuel HOT machines?! That just seems like asking for trouble, man. Fuel vapor near a hot engine block is how you get a fire going real quick. I'm honestly shocked that was ever standard practice where you were.