Platelets/thrombocytes need a trigger to aggregate. When and where clotting is needed, there is a high concentration of molecules that bind to a specific region of the platelets, the platelets get activated by this signal. If this isn't done local (to close a wound) but global in the whole system, the first symptom is that there is a low level of thrombocytes in the blood (thrombocytopaenia), and the reason is that they clotted (thrombotic thrombocytopaenia). Heparin can do this in rare circumstances, then it is called Heparin Induced Thrombocytopaenia (HIT). The regions antibodies bind to are called antibody epitopes: whenever an antibody finds its matching epitope, it binds there. Not every part of a virus becomes an epitope, the immune system tries to find the exposed regions of the virus, and only regions that are unique (so the antibodies don't attack the body). There is a region on the spike protein of SARS-2 that is sometimes used by the immune system as an epitope for antibodies that is almost identical to the region of the platelets that triggers their activation. This means that there are antibodies that were produced against the spike protein, but can bind to platelets and activate their aggregation. This was shown (proven on the molecular level) with antibodies produced against the spike proteins that were created by vaccines. But there is no reason to assume that the immune system reacts different against the spike proteins of the real virus. There is a random factor: The immune system decides which epitopes to use (about 1,000 for the whole virus, 200 for the spike protein), and the immune system has mechanisms to sort out dangerous antibodies (which can fail). These antibodies get produced in high numbers when the immune system has detected the virus or the spike proteins caused by the vaccine. They almost vanish over time, there are just b-cells sleeping around that are able to produce such antibodies again if triggered (by an infection or by a booster shot). DrBeen MEdical Lectures: Vaccine Caused Clotting - Study Finds The Mechanism https://youtu.be/WsRgRP1Oou0 (my fist Corona ad for more than a year) https://pic8.co/sh/Fz0YnU.png