It was really a momentary issue caused by a water main being opened and gushing onto the road. At the time it happened, there was an inch or more of water on the road, and it was not until a later passage over the place, that I noticed there was a steel hatch covering over an underground service, in exactly the place I slipped.
It is easy to see what happened. Rider comes slowly onto road from mud path, and turns on flooded surface with harsh take up speed control e-bike, right on the flooded, steel hatch cover. Power comes on hard, wheel spins out, rider is turning, back end steps out, geriatric rider falls off cursing, picks himself up and rides off red faced with embarrassment....