: Did it need a critical mass to react this way?
: -Wes

Yup - confine mixed epoxy in a container, and it will generate heat - an exothermic reaction - generates enough heat to make it "boil" and foam up. Had an end pour in one of my boats end up about 4 times the normal size, looking like it was made of sponge...making smaller pours, or keeping the end really cold by immersing it in icewater, helps prevent this from happening. When applying coats, spreading the mixed epoxy out really thinly on a plastic-covered piece of plywood helps - the larger surface area lets the heat dissipate and gives you a much longer working time.