The theory is that it is the same reaction that a tiny percentage of people have to heparin (AZ vaccine in this study but mechanism presumably the same) There is a treatment but imperative that doctor knows that this is the cause for appropriate response. https://www.nejm.org/doi/full/10.1056/NEJMoa2104840

As for the AZ vaccine, we are still learning: https://www.nejm.org/doi/full/10.1056/NEJMoa2104840

The paper details several cases (and index case in more detail) as well as a plausible mechanism behind the thrombotic events. Even better, there are some therapeutic possibilities as well arising from the heparin induced analogue condition.