The egg found at the start was stored near a nuclear reactor secretly because it needed the radiation to incubate. As it neared gestation it caused tremors which destabilised the core causing a nuclear meltdown. Joe's wife was a nuclear technician and was trapped behind a blastdoor/radiation shield.

During a meltdown active rods would be cooled by seawater (since they are on the coast) which would flash boil instantly -She was basically cooked by massive amounts of irradiated steam.

That's what caused it. Thank you.