"Why do people see vegetarian’s as a threat?"
The main reason some people who do see vegetarianism as a threat, are those who has a lack of self confidence in themselves. For others it's a fear of something that's unknown to them, is outside what's normal. Then there are those whose lack of understanding, even though they'll usually proclaim otherwise, of how to eat meals, that does contain meat. Then you will have those who feels it's a threat because of those few, who try to push their personal preferences onto everyone else. Finally there are those who feel it's somehow a threat to either their so called masculinity, or in some cases their femininity.
I'm not threatened by those who are either a vegetarian or even vegan, even though meat, and other animal products, are part of my diet.But then that's just me, and I'm neither a vegetarian or a vegan. Over the years, I've spent a lot of time learning about diet and nutrition, and that made me become a bit more knowledgeable, than the average person. I also know from first hand experience, what works for some, won't work for everyone.