You've probably already seen this because it's the top search result, but here's a great series of articles on the subject. You should read that, but to summarize, it's mostly paranoid delusions. The specific manifestation of paranoid delusions are always culturally influenced; their typical representation today, due to this online movement, are often of constant government surveillance and harassment. Things like being followed by black vans everywhere, every person you meet being a government agent, even government mind control through specialized radio signals. These reflect very real fears, but are nonetheless delusions. The author of those pieces also argues for an element of mass hysteria.