I think it could be a couple of things, or a combination of them.
First, as others have suggested, it could be the lube. There's a bunch of different brands of lube on the market now meant to simulate cum, and these tend to be much "whiter" than the actual stuff (ex: Fort Troff's Cum Lube). I have used some of them before, and can say it's a great aid for filming scenes with the "frothing" effect is part of the main goal visually.
Another way/reason for cum turning this way could be due to hydration. I'm not a qualified medical expert or anything in this area, but from experience I can say that a guy who's dehydrated tens to shoot thicker, more dense loads than guys who are hydrated enough, whose loads tend to be more clear and less "rope-like". It's more common in the PNP scene as party guys tend to not look after their water intake as closely, and Tina will dry you right out.
So odds are in these churning videos that there is either product being used to enhance the visual appeal, or the bottom is loaded with chem cum. Just my two cents.