He managed to run into a tree stump, with a big enough thump to mean continuing wasn't an option.
My best encounter with a tree stump was in the Czech Rep the year before WOC. I was having a great race, maybe a kilometre to go, Kowalski had just overtaken me while pretending to film me. I looked at the map to optimise my approach, wondered what the green X was that and why I hadn't seen it and suddenly I was swan-diving to the ground having hit the stump head on. No harm done, thankfully, besides the bruised ego.