Have to admit I laughed too. I think I can guess what happened though.
One area of the woodland path at the top of several cliff faces on the river's banks - which is accessible from a public bridleway - is particularly uneven with a sheer drop of around 30ft at the side which leads down to the river.
The heavily wooded area, which is popular with walkers, has areas of relative flat before being broken by various tree roots and branches.
Those things steer by leaning, so if you leant to one side accidentally as the thing bumped over some uneven ground, steering it off the side of the path could happen quite quickly. Most accidents happen when people get complacent about something they do frequently.
Devious, treacherous, murderous, neanderthal, sub-human of the West.