How can coolant leak into fuel?

Our 2001 Toyota Camry was running without any issues before this incident that happened one evening when I was returning from work from another city 120 miles from home. After driving for about 60 miles in the freeway suddenly saw steam escaping from the front hood continuously. I continued to drive 10 miles slowly to arrive at a city and finally had got it towed to a nearest automotive repair garage. They changed radiator and thermostat in a day and I drove back another 60 miles home. After I parked in the front porch the car could not be started again at all. When the key is turned on to ignition is heard but does not get the car started. So had to tow the car to the service station where they diagnosed that coolant has been leaking into the fuel and suggesting to change the engine. It is hard to understand how this could have happened?