I think it's not just the amount of inflow into a stock its the herding behavior and the speed that sets up these crazy unnatural movements. Most of these stocks are come back to Earth relatively quickly so they all leave at the same time too.
I wonder how much is due to the average joe getting into trading since it's becoming easier and easier to invest money and as it is it looks like free money.
It is retail investors, but to create the momentum that drives the frenzy you need a lot of cash rushing in at the same time. Like with GameStop it was all over the news, there just isn't the same level of media hype to cause the run up. I bet what tips it into a feeding frenzy is a massive institutional investor inflow which creates enough of an outsized move to then attract guppy retail investors who end up driving the craziness and holding the bag. It's part natural behavior, part manipulation.