I too think that menopause itself probably causes an irritable heart in many, and it is probably that rather than hormone therapy.
But since men and younger women don't turn up in cardiology clinics with palpitations in the same numbers as middle aged women, the finger is pointed at HRT.
It's kind of like saying Metformin has been associated with cardiovascular disease. It's the population using it ie diabetics that have the increased risk, the drug is an innocent bystander.
Whilst I personally have no plans to find out, I suspect that untreated/suboptimally treated menopause and dysautonomia probably make extremely poor bedfellows.
I have heard menopause characterised as a form of autonomic dysfunction, which I quite like - my body became high maintenance at 34, now everyone else's is catching up!