From what I've gathered so far it is from the
threads. Most of it is due to not setting the bottle up as per the video, but that was because the video actually came out 2-3 weeks after the bottles hit the market and most people didn't get to see it. I think most find when filling and the excess in the lid gets on things that it is annoying and is why they switch back. I had the same problem with the old Reo bottles only not as bad because there were more
threads the liquid had to move through, but if the
device was standing vertical during a refill they had the exact same problem. I tend to set mine horizontally when filling nowadays...