Habitat loss, farming practices like switching from small grains to corn and beans, weather patterns, hunting pressure, etc. have all contributed to changes in migration patterns. It's not a single issue thing IMO.
I no longer waterfowl hunt but I remember my hay days were back in the 90's when there was lots of water and a lot of small grains still being farmed. The migration started shifting west as corn and beans started replacing wheat and barley. I'm sure the loss of CRP and drought the last few years hasn't helped either.