For one, Hunter x Hunter fans have always been a more "shitposty-er" bunch than the average fanbase, but if you want a timeline, this is my take.
The Hunterchad thing in specific started as a result of falseflaggers going around and saying that Hunter x Hunter is a "Soft Seinen" and other seinens are just "Hard Shonen". Other fanbases got baited by the falseflagging and threads were made disputing HxH's claim as a seinen, or more broadly that the series wasn't mature.
When the argument got muddied to debating whether Hunter x Hunter was mature or not is when the "Chimera Ant Arc debate" came, as actual HxH fans were defending the mature themes of the story by pointing to the Chimera Ant Arc. This is where all the copypastas initially came from, just honest fans writing well thought out responses and people copy pasting them when the same topic came up, that's why those copypastas don't have "based", "cringe", "Hunterchad", or "xFag" in them.
This is the part where it gets a little uncertain because I was mostly off 4chan (being a healthy individual for once) for like a week.
After a ton of arguments over the Chimera Ant Arc, the argument had escalated with other fanbases to which series was better. Maybe it was falseflagging initially, but Hunter x Hunter fans adopted the "My team is better than yours" attitude of the more discussed shonens and ran with it (largely doing it ironically from what I can tell). This got more and more exaggerated as other fanbases got more and more invested in putting these "Hunterchads" down. This culminated in more and more bait threads, more and more pastas being made, and more people becoming Hunterchads through exposure.
Personally I think its pretty entertaining, OC is consistently being made and I think it breaks up the monotony of the boards.