Bob was really popular in life, or became popular by how he died. Maybe he was a hero, great leader or philosopher, or maybe he was just a really nice guy. After his death, some group he never belonged to - and did not sympathize with - claims him as one of their own. A kind of in-story "Misaimed Fandom", if you will. They are bragging about how he was truly one of them, and use his name to encourage others to join them. He's probably spinning in his grave! However, giving your own real followers an undeserved good reputation is generally not covered by this trope.