I would take a guess that it was around this time that more fanciers concentrated on the so called sprint strains. This produced more speed, but the tried and tested distance birds became a bit watered down, therefore the birds have changed. Peregrines have increased by an untold amount and primarily are based in the west. I know they have spread since then. The entry from the west decreased due to the peregrine threat, therefore the drag to the east had more effect. The route was changed to the east to avoid the peregrines, so the birds tend to follow the drag easier than breaking from it. This is all pure speculation on my part.