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because sooner or later, people will see more value in the prices decreasing than in having them increase, or something will go badly wrong - see how Ethereum took a dip when TheDAO got hacked for example.

But mostly this isn't about reason it's about the gut feeling that these things have got too big, like a balloon that's ready to pop.

You could say the same thing about real estate markets around the world, and stock markets around the world, when they go pop where does that money go? Cryto's are now seen as a safe haven asset