My (regular) fat bike frame broke at the same place. Are you tall and was this a large size frame? Mine broke when I stood up on the pedals: the whole body weight + the pressure applied and transmitted through the chain, pulling the wheel axis frotnwards. I have the feeling long frames are more likely to break this way, and fat bike frames too as the chain stay has to make a big curve to leave room for the tire, which weakens the tube and its ability to respond to tensions which are not any more directed in the ‘right’ way.