Not only could it be almost anything that’s increased in our general environment, but better means to identify specific diseases. Diagnostics and knowledge have advanced in the 30 or so years this study apparently covers, and can account for an “increase” in the prevalence of auto-immune diseases.
In theory, there’s still some diseases that while well understood, HCPs still take excruciatingly long to diagnose and prefer to explore routes like mental health and exclusionary diagnoses first, which could suggest prevalence is higher still.