A bad internal transmission cooler located inside the radiator. Fluid is pumped into it and you could have an exchange of fluids. If the Transmission Fluid looks like "pepto" (pink) you will being going for a rebuilt transmission. Antifreeze reacts with the clutch plates inside the transmission.
possibly you have a leaking cooling line from your trans cooler or radiator
A leaking head gasket is is most likely cause, could also be cracked head or cracked block. A leak in the trans cooler which is located in the radiator would allow trans fluid into cooling system.
A transmission can not run without a cooler, for one. Either the water was already in there, or it got in from a leak in the radiator/cooler tank, or from an external source, like rain or splash getting in the filler tube.
Probably any normal thing like not enough Radiator Fluid or something is wrong with your radiator or a head gasket leak could cause the radiator fluid loss. Does this Help?
i would check the vacuum hose that goes to the vacuum modulator on the transmission the vacuum modulator probably has a hole in it and the fluid is going into the engine and being burned or check your coolant the radiator may have a hole in the transmission cooler inside the radiator so check your coolant and the vacuum hose for trans fluid good luck
Low trans fluid,Trans in need of new fluid and filter,bad torque converter,or the tranny's on it's last leg.
dirty trans fluid might be the cause. have it changed, but not flushd
1 simple cause may be a damage radiator cap or simply low fluid.
broken radiator
Thermostat stuck closed.
TAke it to a garage and have someone look at it. The cooler for the trans may have ruptured and is leaking fluid in the rad and coolant in the trans.
A lot of transmissions are cooled by running copper tubing from the trans to the bottom of the radiator and back to the trans oil pan. Depending on the age of the car and the condition of the system you could fix the problem by taking the car to a radiator shop and having them pressure check the system and solder any leaks found in the trans cooler system. Alternately, get a used radiator from a junk yard and replace your old one. The cooler lines are the only point that transmission fluid can get into the radiator water. UNLESS!! someone poured a pint of trans fluid into your radiator coolant bottle!!!! The first step in diagnosing this problem would be to drain and flush the coolant system, refill it and then in a few days check to see if you have the same problem.