The fabric stretches a little bit each time you sew a row. If you keep starting from the same end each time, the stretch will be multiplied by the number of strips you are sewing. This will cause the material to swerve in one direction. In order to counteract this, you should sew each row in an alternate direction and it will stay more square. You should do this when you are sewing multiple quilting rows or lattice patterns as well.Hope explanation is clear - all the best with your project.
If I take a break, I generally put a small safety pin at the end of the las row that was sewn and I know to sew in the opposite direction from there.