By default, a keyframe is in place at the start of the timeline and at the end of the timeline. When you repositioned the text, the position is being interpolated by the software generating a path between the new position and the position at the end keyframe. To correct for this, you need to go to the start of the timeline and click on the box at the left of the position slider and select constant. This will set the text to have a constant position.
You might want to check out some of the free tutorials on our web site: