Why is it considered rude to burp in public? It's largely involuntary and better for one's health than holding all that gas in.
Largely cultural and country specific, but I'd say pople tend to dislike it because of the smell it might make, as well as the noise, which can be quite loud. The same is true for farts and cleaning out the nose, at least in Brazil.
Luckily, you have a bit more control when burping than the others, so you can turn your head down and make sure to cover your mouth, so that the smell won't spread too much, and apologize afterwards. I don't burp often, but by doing this I've never met anyone that genuinely reprimanded me and even though it is not your fault and something that arguably you shouldn't need to apologize for, apologizing makes it clear that you didn't mean to do it on purpose.
This is basically what Maralee McKee, an etiquette expert says.