I see two plausible things the R might have called:
- The herky-jerky movement in the up-and-under may have caused the R to believe that the pivot foot came down -- for better or worse, when things look funny, bad calls can come out to play. (Though it also seems that the herky-jerky may have been caused by the defender bumping him.)
- As others have said, he may have believed the pivot foot moved too much before the up-and-under.
But I'd also note that you say the ref called on multiple occaions. When I coach, I always tell my players that you have to adapt -- whether it is a good call or not, if that is at travel
today, players need to adjust to what the officials are calling. (That is not a defense of bad calls, but the reality of play and how smart players adjust to their enviornment rather than continuing to do what the officials have made clear was going to be called against them.)