Well...here's the rest of the story.
After returning from vacation I went to look at the Horse Walker again.
Found that when Mr Farm Owner poured the concrete slab, the conduit He had stubbed up was left too high, & when they lowered the base onto the slab, it pinched the conductors just enough to make the motor unresponsive to the control, and when reversed, the short was great enough to trip the circuit breaker.
New ditch, conduit, & carflex replaced NMB, & all is good.
Never shorted enough to chirp a continuity tester, but was able to see a short with a meter.
I hadn't seen a motor act this way before, but the one thing I knew, without a doubt, was that Someone here would nail it quickly.
Thanks to ALL who replyed.
