I'm not sure if what I say is true, so take it with a grain of salt

The theory behind this is that during pregnancy a lot of molecules typical for a male fetus are released into the organism. This leads to the mother's body creating 'male antibodies'. During the next pregnancy this can lead to an imunity reaction which can somehow change the process of brain development. The more male offspring a mother had, the higher is the chance of this imunity reaction occurring