One theory as for the increase in peanut allergies is:
In the late 1990s and early 2000s, doctors changed their approach regarding food allergies. They mistakenly thought that delaying the feeding of peanut for several years was the best approach to preventing peanut allergies.
In fact, the American Academy of Pediatrics issued a (now-outdated) recommendation in 2000, which directed parents to avoid feeding baby peanuts until they reached the age of three, especially if a baby was at high risk for a peanut allergy.
Today, doctors know that this approach to delay peanut introduction actually increases food allergy risk, and that delayed introduction was a major factor that led to the sharp increase in peanut allergies.
Thanks to landmark clinical studies, doctors now know that the opposite approach, feeding baby peanut early and often, before they turn one is the best way to prevent peanut allergies.