I don’t know for sure; but if I had to guess I would say, “Spike Proteins”....which the vax produces and which are implicated in many covid symptoms.
The vaccine teaches the immune system to attack anything with spike proteins (found only on the virus). Humans don’t have anything resembling spike proteins on their cells, that’s why the spikes were chosen as the target. But in extremely rare instances if some did, and we don’t know if that’s the case, the immune system would start attacking those cells.. on whatever organ they were present. That’s one theory. But we don’t know what the actual cause of heart inflammation is.
62 cases out of 5 million who were vaccinated in Israel. That’s 0.001 %, or one thousandth of one percent.