I'm not saying that Day Care causes it, I'm saying that spending too long in Day Care prevents the compensations that were so often made in the past. That may account for the increase in diagnoses. Interactions with other kids may actually accentuate the autistic debilitation. Kids can be mean.

As for causation, I'd be looking at media influences since they're the easiest thing to control, diet (nitrites, HFCS, msg, hormone-laced meat, etc.), and genetics.