I think he's asking along the lines of why the creeps return back to their spawn point.
There are several ways to solve this, none of which I have test out but in theory should work.
1 - In gameplay constants, edit the neutral creep guard range or something along that line. You might want to have a far aquisition range.
2 - Make them not neutral, but owned by a computer controlled player.
For me I just make a periodic, but this causes jerkiness. You can do what some other people do and create checkpoints short distances from each other so they move from one and receive a new order to move to another. | 
