how did anti freeze get in transmission fluid

expansion tank thermostat broke caused exp. tank to burst. no oil in radiator. I changed tank and therm. put system bak together and filled with coolant. checked trans oil it had water in it and now I hav a light shaped like a gear and transmission is stuck in 3rd gear I empty out about a gallon of water . filled with freash oil . disconected battery and reconected light is still on and trans is still stuck