Interference?
 

B1 lays down a bunt up 1b line, actually well inside fair territory. F2, playing the spin, runs up the line in foul territory, parallel with batter, then collides with B1 when turning into fair ground. F1 fields the ball cleanly and sails the ball over F3's head.
F1 actually had the better play on the ball all along. Collision occurs maybe 10' up the line. F1, F2, B1 & ball all pretty much lateral to each other at time of collision.

If the umpire judged F1 as the primary defender to field that ball, this is OBS by the catcher. Protect B1 to 1B. I don't understand the last sentence as if it is true, how could ther been a collision :D

I think he meant "parallel." :)