How does trans fluid get into radiator?

You will notice two lines going to your radiator from the transmission. One is a service line and one is a return line. The radiator has a transmission cooler built into it. If you have trans fluid in the radiator chances are the trans cooler is leaking. A little trans fluid in the radiator will not affect engine cooling BUT as soon as the pressure drops in that transmission you may very well have radiator coolant sucked into the tranmission and that will ruin a transmission in a hurry. You need to get this checked out as soon as possible.