Not the bike's fault, they'd have fallen off anything.
The rule is stay on the flat black bit, not the bumpy green bit. The fact that this road is many layers of tarmac thick, rather than grading each one before a new layer goes down can't help at all, it just makes it further to fall.
When I went round the North Coast 500 (on my HD) I watched a German biker find out how deep Scottish ditches really are (it went in up to it's handlebars) and stopped to help pull his bike out, the next day, going over the cattle drover's pass at Applecross in thick fog, I came close to falling off the edge of the tarmac, like these guys, when I met a Rolls Royce coming the other way. Not the place to do it:
https://www.dangerousroads.org/europe/scotland/60-bealach-na-ba-scotland.html