Most (all?) vaccines have something in them to make the immune system recognize that there's something there to fight. (It might me called an "adjuvant" or something like that - I'm not the pharmacy student, so I'm probably getting the word wrong.) It's included in the vaccine that you get from the manufacturer.

That was fine, when you only had a very few vaccines to get. But now kids are supposed to get something like 18 different vaccines. Well, doctors know that the mom isn't going to bring their kid in 18 different times to get vaccinated. So they give the kid five different vaccines at a visit.

So the kid gets five doses of the adjuvant (or whatever it's called) at once.

For some kids, that's enough to make their immune system hyperactive. And behold, they develop autoimmune diseases.

As I said, this is a plausible theory. It's not proven. But it makes some sense, and it explains what changed.