"Why do people get so triggered if someone chooses to not eat animal products?"
There are three reasons why people become triggered by those who chooses nit to eat any animal products. The first is they were conditioned from childhood, to be judgmental of those whose dietary preference excludes the eating of any type of animal products. Their parents raised them in a way that if someone had chose not to eat meat, or any animal products, that they should react to it, in a negative manner. Some actually though manage to mature enough, that they will move past it. Of course in most of these cases, the parents have used as mix of science pseudo science, and myth, to teach their children what they believe is the truth. However some upon adulthood, and once way from their parents, and usually over a course of many years, can work past some of that childhood conditioning.
The second type are those who have by self conditioning that any diet that excludes any and all animal products is wrong, because they have decided that they are better suited to judge others diets, and what is a healthy diet. Often these are the self ascribed grandmaster aged ancient all wise all knowing experts. This type of self conditioning will usually be found to have had its base roots the beginnings while they were still in their teen years. This type rarely ever will get past that self conditioning, as they've made up their minds that they're dietary science experts, and yet they don't have anything beyond a high school education. But yet they know not only as much but believe firmly more than those who have spent decades working in the field of nutritional research. This type will refute anything that doesn't fit what they've determined to be the facts. Of course there are those from the first group, that has further ingrained into themselves the conditioning from their parents.
The third type, and in many cases are of the worst possible type, are the former vegetarians and vegans,who are those who suffered from failure to thrive on a vegetarian diet. They can make some of those who were never a vegetarian or vegan, look like children, when it comes to being judgmental. After suffering from a failure to thrive, they see themselves as being the consummate experts in the field of diet and nutrition, and commonly the most qualified to make and pass judgment on others diets.
Also in many cases with any of the three, they don't have a degree in nutrition, many times tries to come off as knowing not only as much, but more than a registered dietitian, and someone that works or had worked in the field of dietary nutritional research. it's also these type who can trigger my becoming very vexed, or triggered, with such an attitude. That also includes those who are vegetarian and vegan as well.
This is not to say that there aren't any who are triggered by those who chooses to eat animal products. As pointed out there are actually several, whether currently active or inactive, that are highly judgmental, where diet and nutrition are concerned.
Now here are some previously made statements, that hold a lot of truth. Also so as to not be calling them out, no names will be mentioned. Yes I'm cherry picking, but they are accurate enough, that they can stand on their as valid answers, that would be worthy of best answer.
"it doesn't effect me at all."
"Sadly radical people are the ones the spot light usually shines on."
"My main theory is that it is defensive. It's probably just a small percentage of people who do that."
"The same reason why vegan care or get trigger about people who eat meat or animal products, they have their own personal view of what is right or wrong for a diet."
"most people will at least make judgments without knowing you."
"One bad egg tends to leave an impression most people don’t actually hate vegans/vegetarians it’s just annoyance at past experiences"
"Because they see veganism and vegetarianism as aberrations from the norm."