For a long time, we've known there is an association between birth month, generally a winter birth, in the months of February and March especially, and the development of schizophrenia. So, there's been all kinds of thoughts to why that might be.
But, most of the research right now is centered around viruses. Viruses like Toxoplasmosis, Influenza, Cytomegalovirus virus and their relationship to schizophrenia. So, it's thought that maternal infection near the time of birth or an infant infection just after birth are thought to be related to the development of schizophrenia in the brain. Those infections are more likely to occur in the winter and so that's why that relationship is thought to occur.
The other possible ideology is vitamin D. In the winter, especially in northern climes where this association is found, we tend to get all covered up, we stay indoors, we don't get as much sunlight and so we have lower levels of Vitamin D.
That is postulated to be a reason why the brain development might change and give rise to schizophrenia. This presents the possibility that better prenatal care, so Vitamin D during pregnancy, protection from viral illness, might lead to better outcomes and less risk of schizophrenia. But, that research still needs to done.