And so we come round again to autism. . .It seems that a combination of genetic, environmental, neurological, and inflammatory factors contribute to the development of autism. Today I would like to focus specifically on the inflammation and other evidence of nutritional contributing factors. 
The best evidence of the actual inflammatory damage comes from the work of some neurologists and pathologists at Johns Hopkins. . . They found that the most striking differences between autistic and normal brains were loss of the purkinje cell layer in the cerebellum, and also a marked activation of the microglia, which are cells in the central nervous system that are central to the inflammatory response. . .
The researchers in this paper spent a great deal of time reviewing the statistics and making sure every last variable was accounted for (including age of mother, previous children with autism, age of father,education of everyone, etc.).  They didn't spend a lot of time trying to figure out why the pregnancy interval timing would matter in the discussion, but they did briefly mention possible depletion of folate,omega 3 fatty acids, or stress (it is, obviously, very stressful to have a young baby and to be pregnant at the same time).   . .
Since, during pregnancy, the fetus will tend to suck whatever nutrients are needed straight from mama, whether she can spare them or not, it is sensible that a nutritional explanation could account for the increased risk of autism in second children when the pregnancies are closely spaced.  By nutrition, I mean anything from deficiencies in omega 3 fatty acids, to minerals such as zinc, magnesium or selenium, or depletion of the B vitamins, including folic acid.  My suspicion is that small differences in nutritional status can increase risk at vulnerable times in the development of the fetus.  The absolute causes, however, are yet to be discovered.