I certainly was moving, and I know I was moving quicker than a snail. And far too quick for a snail to climb up my tyre and jump on to my derailleur. So how does this happen?
As you can see from the grass seeds on the bike, I did go through some particularly long grass.
The ONLY explanation is that somehow it fell or was knocked off a bush or the grass in someway and hitched a lift!
Now you can move a long there’s nothing to see here.