The possible reasons are all pretty bland; gravitational lensing, nebular refracting, or they weren't stars at all but rather asteroids (with a vector of motion in-line to that of the LoS of the observation).
It's not like these stars had ever been catalogued before the first plate, so its not like these objects were long-standing unchanging phenomena that suddenly disappeared. These are hour-long transients of which there have been hundreds recorded.