Worn clutches and bands/brakes - when these parts have worn out, they cause slipping which generates excessive heat.
Faulty solenoid - Faulty solenoid can cause wrong application of fluid pressure that can lead to slippage of clutches followed by overheating.
Hot weather - Ambient temperature can raise the internal temperature of transmissions especially those type of transmissions that do not have a transmission cooler or heat exchanger.
Driving habit/condition - A transmission can overheat when it is subjected to severe driving condition like harsh driving, prolonged stop and go driving and driving in terrible off-road terrain.
Faulty torque converter lock-up system - Torque converter lock-up is meant to limit transmission slip at certain speeds. If the lock-up system is faulty, the transmission could overheat after a prolonged use.