I don't know either. Histamines are a puzzle, I think it was 'laughingdogs' that worked on histamines for a while if you go back looking for her posts.
One way I've heard it explained is that if you undermethylate, the body must constantly produce adrenaline to reduce the histamine levels keeping you in fight/flight until exhaustion.
I get the tachycardia and post nasal drip too, although since going after gut dysbiosis a bit more, those symptoms have subsided.