Actually, it is part of a Mast Cell issue, but that's what herxing is - a more minor issue with mast cells. A more serious issue will have a person starting to feel an anaphylactic type reaction.
The Epsom salt soaks can release too many toxins from the tissues, and dump them into our bloodstream, and that's where our bodies start to have issues if we aren't good detoxers. The body begins to get overwhelmed with the toxins in the bloodstream and simply can't detox them out fast enough. The "common name" that has been put to this is 'over detoxing':
www.tiredoflyme.com/4-reasons-why-you-still-feel-bad-despite-detoxing.html Making sure that your liver is well supported, as well as the kidneys, and the bowels will go a long way towards settle down a herx, but binders are really good during times of over detoxing.
There actually was a time I could soak in a tub filled hot water and Epsom salts/peroxide, so I'm not sure what changed, but I can no longer use them and have to stick with only foot soaks.