Too much or not enough back twist. After you make the first loop, count how many twists it takes to get the bundles straight, then add that many back in the opposite direction.
After you get the second loop make, grab both ends and "snap" it a few times and look at the bundles--you should have little to no twists in them. If you do, take note on which way they are twisted and make the appropriate adjustment to your backtwist. Just like most everything else in this sport, consistency is the key.
Chad