I rarely use the rear brake (right foot), and then mainly for low-speed manouvers. And if he was going down a motorway he quite possibly didn't need to change gear (left foot).

I suspect he arrived at the junction and tried to do one of the above, or (more potential for amusement) stopped and tried to put his foot down.

Reminds me of a story I heard. The storyteller was pulling up at a junction, behind another biker. The other guy stopped, and promptly toppled over. He helped him up, and the embarrassed biker explained that he'd been riding with a sidecar for years, but had just removed it, and kept forgetting to put his foot down.