Collaideout of curiosity, any tf2 linguists out here wanna explain when we started calling americans burgers?
also someone should make a urban dictionary post about it too
Pejoratives have long been used to distinguish groups of people on superficial value or that groups most outstanding feature. When comparing cultures are face value, the easiest assets to compartmentalize and distinguish against would be food, dress, language, and traditions. Most non-Americans already know the stereotypes of being fat, eating unhealthy foods, being mobility scooter bound, being gun crazy, speaking one and only one language, being racist, and having incestual tendencies, and various insults and slurs can and have already be derived from these stereotypes.
The term "burger" shares the same genetic prose as the term "kraut", which was used in America and the Anglophone world at large to describe Germany and German speakers of any country, which was most popular during WW2 and the post-war and early Cold War periods. While there are obvious differences between German speakers in different countries and even with Germany herself, the term had such popularity because it associated an entire umbrella group of people to a uncommon and insignificant dish in American cuisine; sauerkraut. The terms usage at the height of its popularity carried a very heavy negative connotation, as many Germans and German speakers faced hostility during WW1, WW2, and the post war era. Sentiments have for the most part died down in our current day and age, and the term, while being dated and annoying, is most often used in friendly banter, since most Americans don't really carry any resentment towards Germany or any European country for that matter.
"Burger" is used in much the same way. Its an umbrella term for all people living in America and Canada, regardless of race, age, and socioeconomic standing. Hamburgers are the quintessential American food, you could walk into any bar, restaurant, fast-food joint, or any other place that sells food, and you would have an almost 100% chance of being able to purchase a hamburger in one form or another (unless that particular vendor specializes in something that would traditionally exclude burgers and other 'American' foods; pizza joints, ethnic restaurants, ice cream parlors etc). The burger in American culture is almost as ubiquitous and symbolic as the bald eagle, Ford F150's, and police brutality.
Correct me if I am wrong, but I am almost certain the exact term "burger" used to describe Anglo-speaking Americans and Canadians comes from fucking polandball comics. Polandball comics, for the uninformed, are web comics that use the culture and outwardly appearance of a particular country as main characters, the purpose of the countryballs is to represent the macro-generalization of a particular country. Since the comics are most often written in English, non-English speaking countries are typically portrayed using broken English, usually involving the linguistic features of that country's common language (ie, Russia, Poland, and other Slavic countryballs not using definite and indefinite articles when speaking English, since most Slavic languages lack this feature; China- and Mexicoball using xixixi and jajaja instead of "hahaha" respectively, etc). Since most countries in the comics are displayed as being dumb, ignorant, our out of the loop, stereotypes and pejoratives are often used for comedic effect. Since burgers are a staple American food that has seen a rise in popularity overseas thanks to America's cultural imperialism, most countryballs refer to Americaball as "burger".
To answer your question of when tftv in particular started using burger, that's harder to pin down, since people online have been calling Americans "burger" for sometime now, and it seems to have the most usage on sites like reddit and 4chan, where various other memes have incorporated the term "burger". I'd probably say from my own experience lurking on this website that "burger", "leaf" and "euro" became most popular around the time ESEA stop supporting tf2 in NA. Now that there was no unifying competitive 6v6 league in the NA scene, Americans and Canadians (and I guess Central Americans as well) were left with slim and grim options as to the future of their scene, while South American, European, and Asian leagues seemed to face no trouble at all. And since people love to kick other people when their down, I imagine the usage of the term, while not inherently negative or mean, started being wide spread to banter against poor burgerfags without a comp scene and forced to work sigafoos plantation.