Could it be that the hub is simply a cheap hub? If one has to I.D. the hub it may be a no name that simply doesn't stand up to a quality name brand hub. Not sayin it in a bad way but u get what you pay for. We all start somewhere in our budget. Replaceing the hub with a better quality one would prevent this in the future. On a fixied gear the drivetrain is so much more important than a freewheeled bike.