I think its more of a case that you filled the tank all the way, nearly to the brim, with gas from a tank in the ground
the gas coming out of the hose could have been very cold, maybe 5C
when you filled the tank with the engine hot, and sitting in the hot sun, the gas quickly warmed up and expanded, forcing it out of the cap
I was not aware that gasoline "boils" from 100F to 400F - the definition of boiling is when the fuel vapor = 1 atmosphere in an "open" container.
If the container is sealed the pressure will increase as the temp increases, raising the boiling point.
I guess it could be a little of both.
I never fill the tank all the way to the brim unless Im riding and know I will be going at least another 20 or 30 miles to pull the tank down a few quarts, before I stop.
Now that I typed that out loud, that is not good either, because I might intend to get on the bike and ride another 30 miles, but who knows, something might happen a mile down the road causing me to stop... I guess I should stop doing that.
Filling the tank all the way, and then parking the bike for the day is the worse case situation.