3 ft running lane

[bbman]

Could someone explain this nfhs case play?

8.2.5 SITUATION B: R1 is on third, and the batter receives a base on balls. The batter-runner runs to first completely in fair territory (outside of the three-foot running lane) the last half distance to first. The catcher throws the ball to first and the throw strikes the batter-runner. RULING: If the umpire judges that the batter runner interfered with a legitimate play by the catcher, she should be called out.
COMMENT: All bases must be run legally, even awarded bases. (8-2-6; 8-3-11; 8-6-10c).

What kind of legitimate play was the catcher making?
Why was she throwing to first when the batter received a walk (and the runner wasn't even to first base yet?

Thanks?