From the way that you have explained it. I think I would have called nothing. Tough-luck. Defensive error. Play on.
If the original collision caused the runner to be knocked down then I think the coach was probably correct - obstruction, given that the runner was avoiding contact by running far enough away from the fielders.
Obviously the defense was having some communication problems.
It was probably a Had To Be There, HTBT, play.
Just my opinion.
