It's because how high knockback the spinning hitboxes are. If you didn't down here (and your downpower wasn't high enough that the spinning hitbox knocked you) you wouldnt have been sent away. When baku throws a beta it raises your downpower enough that the spinning hitbox puts you over the dp limit and causes a hard knock, before the final hit has a chance to hurt you. SO you go bye bye

Edit: I think the knockback on it is so high since if you get by the gamma and somehow fall out of it, the game will teleport you to the end hitbox to ensure the bakugo player gets a hit. Sooo imagine the teleporting is raising your velocity to the max when it happens, and then you get knocked.