umpire makes the call

[rbrown]

SDituation: my daughter hits a softball over the left outfield girl's head and rolls into the girl's soccer field. My daughter has rounded third before the outfielder picks up the ball.
The girl's soccer team is warming up by jogging in their field where the softball end up. The soccer girls stop jogging, get out of the way, and let the outfielder pick up the ball and throw it into the field of play. By this time my daughter has already scored. Why did the ump make her return to third base saying it was interference? Everyone including the other team felt the umpire made the wrong call. There was no fence between the sofball field and the soccer field.

Thanks