why is my transmission leaking fluid when i start engine

You could have a front pump seal leak which eventually seals when it heats up. Where under the transmission do you see the leak?

Does this happen after the vehicle sits overnight or for long periods? If it does, then it could be a result of the Torque Converter draining fluid back into the transmission, then blowing fluid out the vent at startup. You can verify this by checking and noting the transmission fluid level on the dipstick after you park the vehicle and turn it off for the night. Next morning check the fluid level again before starting the vehicle. If it is reading way above the previous nights reading, thats what's going on. You can fix this problem by having a one way check valve installed in the transmissions return cooler line. Let us know Mark@katzmanauto.com