It sounds a lot to me like this ump needs to update his knowledge on the rules regarding base runners. As coach james mentioned, if "backpedaling" were illegal, there would be no such thing as a pickle. The ump is making stuff up with this call.
A straight line between bases is irrelevant for base running. If it mattered, every rounded-off turn would be illegal. The running path/lane is 3' to either side of a straight line between the runner and the base at the moment a fielder is attempting to make a tag on the runner. The runner cannot leave that lane to avoid a tag. Before an attempted tag play, the runner can run anywhere they want - even circles in the OF.
