The issue in all fields became significantly worse as developing countries decided their universities needed to become world class and demanded more international publications for promotion. Look at the universities in the table in the paper and you can see which countries are clearly gaming the system. If your local bureaucrats can’t tell which journals are good and which are fake, the fake journals become the most efficient strategy. Even worse, publishers figured out that if you can attract a few high-citation papers, your impact factor will go way up (it’s an arithmetic mean) and your fake journal becomes “high quality” according to the published citation metrics!

Math is particularly susceptible to this because there are few legitimate publications and citation counts are low. If you are a medical researcher you can publish fake medical papers but more easily become “high impact” on leaderboards (scaled by subject) by adding math topics to your subjects/keywords.