One possible theory that's not mentioned in the article is that CS has probably become less approachable for those who don't enter college already somewhat versed in at least the basics. Something that is pretty much not true at all for any other STEM major.

I don't do a lot of programming myself, but I took an intro Algorithms in Python MOOC from a top school a while back. I think it's fair to say that, had I no appreciable programming/using a computer background, the class would probably have been impossibly difficult for me.