Part of the answer is how youtube works- youtubers need views, so they need to generate a story with change and drama. "Yup, still vegan and doing fine" isn't a story that is going to get views. Many response videos were "yup, still vegan" and they didn't get millions of views.
Many youtubers did response videos, I like Mic the Vegan's response videos. Unavoidably, trying to figure out what happened is a lot of speculation because the youtubers cite health issues, not necessarily supported by confirmation with doctors. Some of these health issues very plausibly relate to some of the extreme and unusal things they did, like drinking alcoline water to the point that it probably had an effect on their digestive system. That is just one person. Each youtuber that is prone to trying out extreme things did something different, fasts, eating just one food for a prolonged period of time, etc. The only pattern is that these are out of the ordinary practices and probably made for good stories. I don't think all of these youtubers set out to insincerely try a health fad for views, but when you get so many views that youtube starts paying your mortgage, I can understand how folk might naturally fall into doing crazy health stunts for the camera.
Another way you might want to look at this question is why do any veg*ns stop. This article says, 43% of those who stopped stopped because it was difficult to stay "pure". i.e. logistics problems, for example when eating at restaurants, with others, etc.