I unfortunately was unable to replicate it in other playthroughs, but what specifically happened was that, whilst pedalling correctly, one side tilted as I assume to be intended, but the other side did not. As I recall, I even held down on the left side, which did not animate, and watched as the bike leaned into itself and crashed, despite half of the sprite still being upright. A weird phenomenon! Only happened once, so nothing game-breaking, but certainly peculiar.