It's a defensive backlash against the "holier than thou" attitude of most vegetarians, and particularly vegans. That stereotype didn't manifest out of thin air. You can make the argument that the aggressive "veggies" are a small but vocal minority... but even if that is the case, it's irrelevant, because by definition the most vocal get heard the most.

Look at some of the other replies in this thread. "Vegans are on the right side of history." For every reasoned, logical, and calm vegetarian explanation of why it might be a good idea to limit your meat intake for x, y, and z scientific and moral reasons, there are five who barely stop short of insinuating that you are literally Satan for consuming meat.

I think the biggest problem most vegetarians/vegans have is very few of them appreciate our place in history and evolution, and are only interesting in feeling good about themselves in comparison to other people; they spew asinine shit like "vegans are on the right side of history," without realizing or appreciating that we are only just now entering an era where vegetarianism is feasible on a large scale while keeping populations healthy. And we're not even there yet... but in order to feel superior to everyone else, they feel inclined to tell everyone they are on the wrong side of history.

And in my anecdotal experience as someone who knows and is friends with a fair amount of vegetarians (only one true 'vegan' though), most of the people inclined to respond in this holier than thou way don't even grow their own food, or purchase it from communal shares or local farmers/community markets. They're corporate consumers like the majority of everyone else. They don't realize how much work actually goes into feeding a family on home grown food with a very small carbon footprint (in case you're not aware: it's a shitload of work). It's not even remotely possible when considering large populations, so we have to sacrifice carbon footprint in order bring this lifestyle change to the masses.

It's a myopic perspective that has no appreciation for the granular details and complexities of the situation.