Often there's some kind of event that starts the pop, like when Elon Musk slashed the headcount at Twitter and everyone else saw that as a signal that the hiring frenzy was over and it was safe to start cutting staff. There was lots of discussion before that day about how out of control tech hiring had become but the money was flowing and everyone wanted to look like a great place to park it.

My guess as to what happens here is one of the big players showing that they're cutting back on capacity in a significant manner, which will spook the investors who know a bubble pop is coming but want to time their exit as near the top as possible. By the time the pebbles notice, the avalanche will be in full roll.

That's part of it... I would say the real end is closer when you start seeing F*ckedCompany style articles about $Startup closing doors on employees, or hardware repossessions. It was pretty common through later 2000-2001, the finial pop imo was following 9/11 well after a lot of the startup market fizzled, but a lot of more traditional employers started cutting projects/staff too.