The article said it was removed via a minimally invasive craniotomy, which, if accurate, would mean the clot was inside the skull. I’m the wrong kind of MD to trust my expertise on the details, but subdural hematomas come to mind, a clot beneath the outermost of the 3 layers of membranes between skull and brain. They can occur as delayed sequelae to a fall or other local head trauma and can be serious if not treated in time. Reagan had a subdural hematoma removed while President and did fine. If there were no history of trauma than other potential causes, like past tumors, rise as possibilities. Hillary’s transverse sinus venous thrombosis also crosses my mind, but the described location doesn’t seem to fit that.
He had a melanoma removed from his left temple that was pretty deep. But that was a while ago so I don't know if the two are related.
I would’ve thought he was a blood clot risk considering his breaks from the plane crash.