I try to avoid going there, but from what I have seen its somewhat of a snowball effect. You have someone post about some crazy YOLO move they made and see massive gains on. The next time someone posts something a little crazy more people do it. And a few get real rich again. And eventually you seem to have a lot of folks who have enough money to move the needle. And once it starts moving, the FOMO piles on and it gets out of control.
Add on top of that algorithmic / momentum trading following trends.
You have stuff like this person, top 3 post of all time on there from Friday (2 days ago): https://www.reddit.com/r/wallstreetbets/comments/l2x7he/gme_... Based on the way things went today, they had to be up over $10M maybe even $15M for a minute there. People see that and get jealous. Say they cashed out. Now they have at least 10x the money as before and can move the needle even more. Also worth noting initial price of what they have now is almost $800k. So while the description is about gambling little money, high stakes gambling pays off for some. Going back in history, this person seems to have started piling in to GMU in June of 2019, and kept going since. But no real indication of how much they had to start with, or anything like that. Did they have some other bet that they played before this? No idea.
Edit: Also, forgot to mention if you move the needle enough short sellers end up getting margin calls driving the price further up as they get liquidated. https://markets.businessinsider.com/news/stocks/gamestop-sto...