He was trying to reach a particular place in the cave but wasn’t where he thought he was. Both the place he was trying to reach and the place he actually was, are extremely tight squeezes that are literally impossible to turn around in. The difference is, the place he thought he was, has a large cavern on the other end where you can stand up and turn around. Once he realized he wasn’t where he thought he was, his only real option was to move forward and hope it led somewhere with more room. Falling into the hole the way he did was largely an accident in pursuit of that goal.