Transmission. At take fails to change gears and accel. From this caused leak!

What is your question?

Merc Sable was slow to accelerate and change gears at take off. I told my sister to park it and turn it off and then noticed transmission fluid leaking under vehicle. Tomorrow im going to get a rear/bottom gasket for the trans since after cleaning it off, refilling it and observing that is where it appears to be pooling and coming from. My question is what may have caused this to happen?