Part of the problem in the UK (so may not be the same in the US) is that there is additional funding and support for kids with diagnoses like ASDs so there is a big incentive for schools and parents to push for it. It's driving a whole approach of medicalising behaviour. Kids who in the past would have been simply regarded as a bit unusual and who a teacher would have had to just cope with are now being given medical diagnoses and possibly additional help.
As discussed in the article what would be interesting to see is more detail on the distribution of ASD diagnoses, in terms of where they sit on the spectrum. If there is an increase the diagnosis of severe autism (the kids who would reasonably have been diagnose as autistic 30 years ago) then that would suggest that there is some environmental factor at work. If, on the other hand it's mostly high functioning and borderline then it seems likely to be mostly down to diagnosis.
While I'm very much in favour of education being better able to deal with kids' differences, I'm not sure medicalising it is the way to go.