OBR 8.05(g) - The pitcher makes any motion naturally associated with his pitch while he is not touching his pitching plate.
Since RHP tend to place their right foot on the rubber first and deliver by moving their left foot first I suppose that could be interpreted as a natural pitching motion. If this is the thinking the umpire may have poorly explained that this was his rational.
Without seeing the play it sounds like a reach - but it is the most plausible thing I could see in the rules.