At the time that Brock and Hall were committing, Iowa was set on taking one back (Goodson). Since then, Kyshaun Bryan has left from that position and others have transferred from other positions, opening up scholarships. That open things up for taking another running back. Iowa also learned in that time that IKM could be injury prone.
I have not heard of any current scholarship running backs considering leaving. I think it's more about having more room and running back being a position that can be added to as a result. Recruiting is very fluid. Iowa is now looking to take another tight end with Fant going and Hockenson possibly going. That's changed since the beginning of the season.