He and his group thought they were in the birth canal, which is a very tight passage that opens up in parts where you can stand and turn around and go back.
But they weren't at the birth canal. So, Jones was in the lead thinking "it's gotta open up down here farther if I can just keep going."
However, he was in a part of the cave that didn't open up and was eventually stuck head down at a 90 degree angle and couldn't get out himself or pulled out because of the angle.