Think of it this way. There's a finite range of motion on the knob. It has stops at both ends. It's like a throttle, 0% to 100%. When using it for trim, the "up" and "down" are like a throttle as well. When you put auto trim on it handles it for you. When you turn it off, the trim doesn't just stay where you were under auto, it re-sets it to where the knob indicates.
So if your trim knob is 100% (I'm guessing "full up") and you hit auto, then turn auto off, it will (as fast as the trim can move) go back to "full up".