I think it is just a humorous reference. St Vitus' Dance (or Sydenham's Chorea) is a symptom - jerky movements - associated with Rheumatic Fever, which is caused by streptococcal infection and damages heart valves etc but in this case is affecting part of the brain.
So St Malvern's dance is presumably just the Morgan community's term for steering shimmy. It seems to happen quite a lot though! I had steering shimmy once on my Discovery while towing a heavy boat and braking at speed downhill, despite it having a steering damper - perhaps it was St Lode's dance?
