I'll assume you mean well but that you're just not super well versed in communicating over text.

You're coming across as snarky and condecending. The easiest example to help pinpoint a clear example is when you asked if the person you responded to spoke English as their primary language. From a very charitable perspective it could be construed as a benign question, aimed at helping you understand why you didn't understand each other (since you had differing opinions).

But given the context of an argument, an argument on the Internet at that, the first instinct for pretty much everyone is to assume it's meant as an ad hominem, an attempt at discrediting the person criticizing you by belittling them for their spelling and/or grammar mistakes.

It's also important to know that when you're replying many times in a thread people will likely read more than one of your comments so it's enough for one of them to come of snarky/condescending/elitist etc for that to the color the reading of all your other comments. This is exactly how we handle verbal communication as well as humans, it only takes one shitty joke, instance of poor attitude or a rude comment for everyone in earshot to form an opinion about what kind of person they deem you likely to be. People with a larger sample size of your behaviour might chalk it up to a bad day or a reaction to something someone else did earlier.