If the situation as described is that A is slightly ahead of B then it is Bs responsibility to yeild to A. Not much different that the two players running up the floor then the lead player (A) stops/slows and the behind player runs up their back. Not much different than a player driving down the outside of the lane, veers to the basket and gets clipped by a defensive player (B) running any straight path. Foul on B in all situations. That would be my call based on the rational that no legal guarding position is established, time and distance is not applied because A. no blind screen and B. a speeding player isn't getting screened with no chance of avoidance.
In this case, my imagination would have to call a foul on B or worst case a no call. I cant see a foul on A against who is ahead of a running defensive player that does not have legal guarding position.
