Oddly: this coincides with the period in which leaded petrol has largely been phased out on most countries. The use of lead in motor-vehicle fuel is though to have shaved a few points off IQ by itself.

Much as with some other widespread phenomena that come to mind, it's an observed trend the reverse of what might otherwise be expected.

Lead exposure can damage the reproductive system, so perhaps it’s to be expected if a whole generation of parents were exposed to leaded gasoline.

Theoretically this effect would wear off with the next generation, unless there was durable damage to genetics that gets passed on. A cursory search suggests this may be possible, but I’m not a biologist or geneticist.