The short explanation is insecticides: microtoxins to break the fertility of insects for higher crop yields. These same substances also affect humans, albeit on a different level: slightly lower intelligence because mothers are exposed to these substances and they transfer them to the womb, and they also decrease male fertility.

Children of hairdressers or farmers have slightly lower IQ (controlled for all other variables) than others because the mothers are exposed to similar substances.

There is probably truth in that. It's difficult to say as to the magnitude of the effect however.

They are probably referring to some kind of large-scale environmental contamination, but as far as I'm aware it's quite difficult to make any of these theories fit with the timeline of international historical trends. Typically the differences in rates and magnitudes are apparent across borders and thus would more easily be explained with cultural factors rather than an environmental contaminant that is (largely) agnostic to cultural effects.