Activities are sorted a variety of ways: by activity number, by early start, by early finish, etc. The scheduling software does not let you simply move an activity around for looks. It needs a "reason." That is to say, the logic or the sort criteria put it at the end. Other programs, like MS Project, will let you move activities around and renumber them...that defeats the purpose of unique numbers and letting the logic drive the order.
Try adding logic ties to the activities and then "scheduling." Hit F9. Then, go to Tools-Organize and in the lower right corner you will find a Sort By box. Add Sort by Early Start and then press Organize Now. If the activity you wanted to move is the last in the logic sequence, it will now show up as last on the screen.
Good luck.