what causes coolant in transmission fluid?

The automatic transmission cooler in internal to the radiator. Coolant mixed with the transmission fluid would indicate an internal radiator leak. Replacing the radiator and then flushing the transmission fluid should correct this concern. I would not drive the vehicle with coolant in the transmission fluid, doing so could result in internal damaged to the transmission due to lack of lubrication.

If your tansmission lines run thru your radiaor it could be the radiator