Well first off was the silly lipid hypothesis that states that anything that provides cholesterol to the body is bad and causes atherosclerosis. This is deeply misguided and myopic, pseudoscientific even, but it persists because people don't like to change their beliefs and admit that they were wrong. Second is that vegans are loud and obnoxious propagators of falsehood, pseudoscience, and sometimes outright lies http://www.westonaprice.org/blogs/cmasterjohn/2010/09/22/the-curious-case-of-campbells-rats-does-protein-deficiency-prevent-cancer/ I think that we can say without any doubt that books like The China Study are full of lies, existing to push a doctrine. Oh sure Denise Minger doesn't say that, she is so nice, but I do.
There are people working day and night to perpetuate myths and abuse science for the sake of their nonsensical ideologies. How many of these vegan myths like that humans can't digest meat or that protein "rots your bones" were just fabricated for the sake of hating on a particular food? There are people who simply want to believe something and then pester their friends until they believe it too. Those who don't proportion their beliefs to evidence and act skeptically will be convinced. If enough people say something loudly enough and wag their fingers hard enough it could just become public dogma.
A smaller issue might be that some people just don't like the industrial system, and I don't like it either. But there is so much that can be done about that and the conclusion is always that one needs to eat no animal products at all ever instead of just changing the source. A non sequitur if there ever was one. Is anybody offended by a pasture-based system? Very few, but the zealots make it seem like that doesn't exist, or resort to inane tactics to try to discredit it http://huntgatherlove.com/content/myth-sustainable-meat-and-james-mcwilliams
The nutritional vegan movement is just dishonest. Nobody talks about all of the useful semi-essential nutrients in meat that help the body and particularly the brain function at optimal capacity. Carnitine, carnosine, creatine, choline, and all manner of things that start with C ;) This stuff never gets mentioned, they have been forced to admit that B12 is an issue, but then persist with this ridiculous red herring where apparently there is no need to worry because you won't necessarily become deficient in protein on a vegan diet. And lentils are a good source of iron! Well then. It is sad that parents are restricting nutritious foods from the children that are 100% proven to improve cognition http://www.psychologytoday.com/blog/evolutionary-psychiatry/201202/your-brain-creatine
And then when I tell them (non-judgmentally, even suggesting supplements instead) about it they say "well my child isn't stupid" yes maybe not, and maybe they're getting more magnesium or other useful brain nutrients than the general population, good job with that (I'm serious, anyone who eliminates junk food for their child is commendable), but if they started eating meat their cognition would improve even more! So it becomes obvious that these people aren't generally in favor of doing what's best for themselves, they have an emotional attachment to a particular worldview. Nutrition is an issue for everyone, but the difference with these people is that they profess to care greatly about nutrition, but then belittle important aspects when it doesn't fit their worldview.
However... The paleo movement has been trying to call grains into question, on the same tactic. Let's not just blame those silly people when there are many people saying to never eat wheat because it will kill you. I guess I can blame both, but it is the same propagation of doctrine based on shaky reasoning, unproven thus far. Many people will believe something without evidence because it sounds good to them, and if you get enough of those people then even the more skeptical people can be duped. That is the making of a myth.
Nutrition is the complicated science that laypeople have to understand...or die, and that creates a problem because so many people don't understand science or act skeptically enough. Or perhaps they don't even have time! I know a philosopher, a very smart guy whom I admire, who thinks that it is correct to defer to the conclusions of The China Study and the recent epidemiological "meat kills you" nonsense. And I get it, if he doesn't have the time to read the literature and learn these things for himself deferring to an "authority" makes sense, just like I do if I need to understand something about genetics or other sciences. But whichever authority one throws in with is usually an arbitrary matter. And if they are a liar? Then there is no way to know unless one fact-checks them. And there is a serious paucity of fact-checking.
All in all, a very sad situation. Even if meat was unhealthy it would still be a non sequitur to tell most people to never eat any of it. But the tendency of some people isn't just to recognize that something is their opinion and leave it at that but to force it down the throats of the unknowing. And not all vegans are like that, don't misinterpret, but if only 50% are then it can still add up.