I haven't researched the issue but my guess is that duck dynasty started in 2012 and was gaining in popularity bringing out a bunch more hunters. At about the same time it seemed like we had a stretch of warm winters that influenced the migration. Those two things could be some of it. Ducks can have high fidelity to past migrations. It only takes a few years to have a great impact.

Also, I know several people that travel north to duck hunt earlier in the year. From what I've been told it used to be easy just to go find a spot in North Dakota and ask permission. Now it is overrun. So, these ducks see lots of pressure all the way down the flyway. No wonder that by the time they reach the Mississippi Alluvial Plain (starting in southern Missouri and Illinois) they mainly feed at night and loaf on the refuges all day.