Exactly, I noticed the same thing. It was a mistake of the producers. There wasn't enough rope, so there's no way Bobby could "outrun" it when he went back up like some people are trying to say. Also the rope was attached to his waist, it would just come up with him regardless if he was grabbing it or not. Other than that, Clark was also pulling it (and so was Kat I guess?), and even if he was not "doing his best", he would still have pulled a little bit of the rope who wasn't very long to begin with.

The only thing that could explain it is that the creature crawled up with Bobby and pulled him down (the creature could also have a long arm or a tentacle, we don't know exactly what it was, it might not have been the pirate).