As other posters note some feed-forward defense mechanism requires a larger dose to elicit a response. Sometimes small doses have a cascade effect meaning A->2xB->6xC etc whereas a larger dose would activate a feed-backward mechanism between B and C.

A classic example is endocrine disruptors (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4036398/), a small dose induces a hormonal response, a medium dose the body fights due to detecting it as foreign and a high dose is just regular toxic. Please note this is somewhat simplified but used as an example.

This effect has always puzzled me, so much that initially thought it was pseudoscience (like homeopathy). I mean, in a real U curve, the highest effect is at a zero dose and it makes no sense.

But it looks serious, peer reviewed papers and all that. The main reason seems to be that at a low dose, the chemical doesn't trigger "defense" reactions and can do its thing unimpeded (high effect), on a medium dose, our body starts to notice and dampen the effect (low effect). High doses are simply overwhelming and the effect becomes high again.

I still think there is a lot of pseudoscience around that effect that helps selling a lot of "natural", "detox", etc..., but I don't think all of it is bullshit anymore.