Although turning backwards does seem to be the safest way to get out of the box, it is the least obvious in the eyes of an umpire at times. Instead of it looking like "oh, the ball is coming, I need to move out of the way so I don't get hit" it looks more like "oh, the ball is coming, I'd better let it hit my back because it hurts less". It's like you're letting it hit you in a less damageable way, when you can completely avoid it by turning in a different direction, thus not coming off as an actual attempt to get out of the way. I know this is probably NOT the case in your situation, but that's what it MAY come off as to the umpire. I don't think he should have thrown you out, but whether the player attempted to get out of the way is a judgement call, and is not protestable. The umpire was probably wrong, but you can't really do anything about it unfortunately. I would think in a 10U game though, an umpire would understand that players are obviously trying to get out of the way, they're little kids who don't want to get hurt!