Discussion: I have a theory...

...that it's easier to make mistakes at a walking pace. Like the brain decides 'if we're only going this speed we can't be orienteering so I'm clearly not needed here'...

however I suspect in the specific case you're describing it probably did have more to do with the actual terrain...

I completely agree actually! My brain just wanders to the strangest things while I'm walking. Which in some cases you can get away with, but apparently not in this terrain...