Hunch.
9 times out of 10 they're nutcases driven by outside rhetoric and have no real political opinions of their own. They tend to latch on to something in the final weeks or months before they act as if some great secret had been revealed to them.
The volume of threats s one of the reasons my job includes waking Drs, Nurses, Techs, and receptionists to their cars. Just yesterday the daytime porter warned me to keep an eye out for some clown they arrested in the lobby. He was apparently standing in the lobby yelling about Obamacare funding terrorism by way of all these "A-Rab" doctors. (who are mostly Indian) He's likely been turned loose by this morning. Before I hired in they had a guy rooting through the trash looking for aborted babies. The closest thing they do to abortion is chemotherapy and physical therapy. The rest of the building is dedicated to medical scanning like MRIs, CT scans, X rays, and ultrasound.