Why does a route reverse on itself

I am creating routes along paved roads. Frequently, when I am adding points along a route, the arrows on the route line will reverse themselves. If I keep going along the road, the arrows change to continue in the correct direction. However, there is a short span in the route where the arrows go back and forth.

I can't  see what I am doing to cause this. It does not happen with all my routes.

Can anyone tell me what I am doing wrong?

thank you.