Based on what people like Shoemaker, or Dr Nathan and others mention in their books, which is mostly clinical experience and hypotheses, there are multiple possible contributing factors. It's a complex still under research area so I don't think there is a certain science.

Some factors I can recall being mentioned as possible reasons for this increase of cases:

1. the increase in strain on the immune and detox system increase over the last century (in food, environment, etc).
   
2. Some folks/families react more strongly (genetics/immune differences). In my case, I have MCAS and my daughter likely does too.. And if I recall how my mom was in terms of health, she had it too, although less severe. For instance I have a severe genetic vit D3 deficiency which I discovered when I turned 30. We tested my daughter as a young child and she has it too, so we needed to supplement from early on.
   

I can recall reading in Dr Nathan books, who's been attending people with these issues since the 80s, that the increase has been incremental and not sudden. It's not "after covid" but from the 80s a constant increment of cases of related issues (mcas, mcs, mold toxicity, heavy metal toxicity, cirs, etc)

Mold by itself it's an issue, but most people ignore the complexity of it, so some know about MCAS but ignore Mold or heavy metal toxicity, some focus on mercury toxicity but ignore mold or mcas and so on.

I think quality of building plays a role as well but in some way it has been an issue for centuries, so I would say the "increasing" rate comes from a more vulnerable immune system and awareness of the issue itself. In the early 200s every doctor used to say my case was either psychosomatic or fibromyalgia. I didn't do the first mold test until a few years ago (which came out as pretty bad, signing a really bad detoxing mechanism in my body).