Haven't researched it much but times of drought and floods in the nesting habitat may have something to do with it too.
From Ramsey Russell at Getducks
"The bag limit on Pintails will be 1 during the upcoming season and it's probably because they're shooting too many in Mexico, right? Nope. That's pure politics. What about the daily limits in Alaska (8 daily) and Canada (4 daily)? Only anti-hunters myths that hunting-related harvests destroy wildlife. We hunters know better. The continued decline of North America's pintail population is due to the agricultural practice known as no-till farming, which is very good for conserving soil but very bad for pintails. Per the following Wildlife Bulletin article (1999): "The steady conversion of grasslands to cultivation in the western Canadian Prairie Pothole Region since the 1950s and 1970s has markedly reduced the extent of safe upland nesting habitat, thereby reducing pintail productivity to levels below the threshold needed to maintain populations. Grain stubble attracts large numbers of pintails to nest in the early spring, and cultivation (of the present-year's crop over last year's residual cover) destroys virtually all nests. Additionally, Research in the 80s showed how nest success of prairie ducks, even those that don’t nest in cropland, declined as the proportion of cropland increased in the landscape because suitable nest sites in scattered grasslands and planted cover were rare and predators more efficient. Most wetlands in the PPR have been impacted by agriculture and more than 85% of the region has been cultivated (Millar 1989)." Source: https://www.werc.usgs.gov/fileHandler.ashx...
#GetDucks, the more you know."