Some units have seperate animations for walking and running, like the Tauren Chieftan. This is why there is an art animation field for both run and walk speed, and the unit will use the walk animation over the run animation if it drops below the walk animation's range.
However, very few units had a walk animation, so Blizzard probably decided to scrap the animation from the Tauren Chieftan to make him fit in with the rest of the race's units. The only way to get the animation back would be to import the model from an older version of the game.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.