If the fuel started to flow on it's own to the higher filter, and the filter is above the level of the fuel in the tank, The fuel pump diaphragm was held down by something and when conditions changed it pushed fuel up.
It sounds like old fashion vapor lock.
Found this:
"The fuel can vaporize due to being heated by the engine, by the local climate or due to a lower boiling point at high altitude. In regions where higher volatility fuels are used during the winter to improve the starting of the engine, the use of "winter" fuels during the summer can cause vapor lock to occur more readily."
