> What is the thing that makes it end?
This is much easier to answer, at least in general, than many of the other "bubble" questions out there.
The growth simply requires a larger and larger pool of new investors. It first started with tech enthusiasts who were really interested in the principles of cryptocurrency and people who also knew how they worked. The was a time when everyone in crypto had read the original Bitcoin whitepaper.
Then it spreads to less savvy technical investors, people working at tech companies but not really able to understand it. This was the first bubble a few years back. The crypto boom could have ended here, relatively harmlessly.
But then a ton of money started flowing into the market and into individuals pockets as well. Suddenly the pool of potential "investors" opened up and crypto also appeared to be much more liquid than before with the rapid rise of crypto exchanges. This is when you start seeing TV ads for crypto and hear of people on reddit, with no technical knowledge or skills, pouring all their surplus income into crypto.
It ends when there aren't enough new "investors" to keep the illusion up, and it crashes as people start to need to liquidate that investment. During the artificial pandemic boom there was money flowing everywhere, people could throw tons of money into crypto and still eat and pay their mortgage. Very soon people are going to need to get that money out, or desperately want to, and then liquidity will be a problem because there aren't any people left to feed into the scheme.
The faster the price drops coupled with people needing that money, the more likely a panic is to happen which will cause a sudden crash.