Some of the newer research is linking a decreased amount of folic acid to increased risk of autism. As well as advanced maternal age, preterm birth, multiple births and even children born less than 1 year apart. If one child has autism, there is a higher likelihood of subsequent children developing it.
If folic acid is a "thing", it could probably be attributed to the current american "Diet" that doesn't really include enough folic acid intake. I'm pretty sure in 1964, people were eating more fruit and vegetables. It is possible the nutrition density of the fruit and vegetables was higher then.
It could be because some black women have more children. One study shows that about 18% of black women in their 40s have 4 or more children compared to 11% of white women. Other statistics are similar, but if the timing of children and the increased risk of later children developing it, these can be valid considerations.
And maybe I'm one of "those", but I kinda feel there may be some overdiagnosis of autism due in part to the range it encompasses. When I was a kid, some kids were just weird (me included). Where kids may simply have been withdrawn or a loners, they may be diagnosed with disorders now. I know when I was growing up (70s/80s) mental issues would be a huge stigma. People would go out of their way to not get children treated by psychiatrists, so probably a good amount of underdiagnosis then.
And probably another "trigger" point. The amount of kids in the foster care system with autism and other disorders (ADD) is higher than the general population. Part of that may simply be because foster parents can earn more money by caring for the 'sick' children. There may be financial incentive to have children diagnosed to get additional help and funding. Children with special needs may be given better access to smaller classes, one-on-one services, extra study help, extra test taking time - these are things parents may want, and could even be pressuring doctors into misdiagnosis to help give their children these advantages.
So, there are many links found related to the development of autism. And there are many reasons for the increased diagnosis.