Usally its from dirty tranny oil plugging up the ports in the transmistion its also due to clutch plates being burned from rideing the clutch or the shift bands in a a automatic are wore out.
What Causes The Transmission To Slip?
Band slipping in transmission caused by overheating
My 81 automatic transmission sometimes makes a clinking noise while going through the gears, it doesn't do it all the time? What should I do to correct this it only has 42,000 miles on it?