I don’t disagree with you that much of medicine is hand-waving and voodoo magic, but arrhythmias due to scar tissue likely have a similar mechanism to other well-studied arrhythmias. Overall I think that cardiology has some of the more scientific explanations of its pathophysiology than other fields. Scarring simply interrupts smooth conduction of impulses throughout the myocardium and makes it more susceptible to arrhythmias. If you want to see a similar mechanism explained, check out this video: