This was done because Swagger is used as a luck-based move in singles, so there is no real competitive reason to use it. The reason for the ban was largely to stop the strategy known as SwagPlay. This is where a Pokemon, especially a prankster user like Klefki or Liepard, used Swagger, Thunder Wave, and Foul Play. Confusion + Paralysis makes the victim extremely unlikely to attack. The Attack boost from Swagger makes them do more damage when they hit themselves in confusion, and also makes Foul Play do more damage when used against them. Smogon considered several options to stop this strategy, including banning Klefki and Liepard (the best users of this strategy), or banning Swagger in conjunction with the ability Prankster. In the end , they simply opted to ban the move Swagger altogether.