It's one theory University of Minnesota researchers are investigating, according to a recent Star Tribune (Minneapolis) article.
The hypothesis is that road salt in roadside soil gets stored in the leaves of milkweed, the food source for monarch caterpillars. High levels of sodium can impact muscle development and even poison monarchs. However, studies have found that modest amounts of salt can increase muscle growth as well as brain and eye size in monarchs, all factors critical in migration.
According to the article, researchers are trying to determine how sodium levels impact monarch's massive migration from points north to locations west of Mexico City, and if there's a direct impact on population levels.