@Step - not that I have any good answers or insight yet, but can you tell some specifics about installation? Gen 1 or Gen 2 (based on "had it about 6 months", I'm assuming a new Gen 2)?
Master valve or pump start relay installed (my guess is no on the pump start relay)?
Do you know which zone was watering when the Rachio was unplugged?
Can you determine from the actual conditions if other zones were actually watered or did water flow from the running zone to the perimeter back yard walls?