Pressure is reduced in the transmission in the higher gears. Once all the hard work of starting all the weight from a stop is done (first and second gear) the pressure kicks back. Once at highway speed, the ability to turn the transmission faster than the engine (OD) is easier. Start heading up an incline and the transmission realizes your hiding some lead here somewhere (25 to ??% more load) and it was "suprised". Typically, an incline is an easy task. Now you've got the tranny huffing and puffing and the heart is not pumping enough blood. Think of smoking your favorite cigar while on a treadmill.

Some vehicles do better than others, it's taxing on all of them to some degree. Changing the fluid more often than recommended is a great insurance policy.