The only alternative I see is DTTB's "perhaps F3 pulled off because the Batter-Runner was about to collide with the fielder ==> interference ", but a tough judgement to make and the ball had to still be fair.
By the way, I learned something about softball last night, from one of the teams:
"If they keep scoring runs, we have to score more."
This was in a game that needed an error limit instead of a home run limit.