Here is how it goes.
In a 4 speed auto if you are driving in D on a climb and the vehicle is in 3rd gear for power, the moment you let your foot off the gas to take a turn the vehicle will shift to 4th gear. Now when you give gas while coming out of the turn it will again shift back to 3rd gear because it needs power to climb.
This scenario of upshift and downshift will continue all way up which is not wise and may heat up the transmission.