So far there have been sudden crashes of Bitcoin - going down 5/10/15% in a matter of minutes or hours.
These crashes are limited to affect most of other crypto-currencies but do not affect much the other financial markets.
There are always ‘buy on dip’ people who provide ‘bottom’ and stop the price from falling further.
The trigger for a major crypto crash could be an escalation of tension between world powers and/or a war.
I is going to be triggered when even a small group of people decide to cash out and exit - convert from crypto to fiat.
In this case people will find that the crypto market capitalization - currently around 2.7 trillion stable coin dollars, is matched by only around $100 billion fiat/paper currencies (dollars,euros, pounds,yens…) collected by stable coin issuers and crypto-exchanges.
The actual ‘slice of pie’ is much smaller than anticipated…