I think the bell that rung at the top was the same bell I've seen in other speculative markets: The main people you would expect to see cheerleading throw in the towel and say it's crazy. That happened writ large at the beginning of 2018. There was one presentation at a Bitcoin conference I went to in early 2018 where this long haired huppy guy went through CoinMarketCap and called out every single thing on there as bullshit and scams.