Softball/running after a bunt
I had a situation where my hit laid down a bunt in fair territory along the first base path line. She was two strides out of the batters box and in order to avoid the fielder moved another step into fair territory. Had she proceeded to get into the base path outside of the baseline she would have interfered with the fielder. She avoided the contact but she did not get into the baseline before the fielder attempted a throw to first and hit the runner in the back. The umpire called the runner out due to runner interference because she was still in fair territory. Was the proper rule applied?
It depends, but I think w/out seeing the play the call was probably wrong.
The B-R can pretty much run anywhere they want until they come to the 3' lane which begins 30' from HP. Your OP sounds like they had not yet reached the 3'lane, they were just a few strides down the baseline (not base path that's a different animal). If that's the case unless they intentionally interfered with the throw there is no call, we don't play dodge softball. It's just a bad throw from the catcher, still a live ball.
If they had reached the 3' lane they can still run outside it,in fair territory to avoid a fielder attempting to field a batted ball or if the throw is coming from foul side of 1st base.
If a body part outside the 3' lane interferes with F3 taking the throw the b-r would be out. They would NOT be out simply because a thrown ball hit them BUT because they interfered WITH THE FIELDER TAKING THE THROW. It is possible for a B-R outside the 3' lane to be hit by a throw and not be out but that's a different question.
"The umpire called the runner out due to runner interference because she was still in fair territory" If that is the umpire's reason for calling the out, the "P" word could have definitely be brought out. That is a mis-interp of the rules.