Investors realizing that they can't make their money back. Right now even if they pay $10 billion for a photo app, they're still expecting other investors will buy their shares at a $20 billion valuation or more later on. All investments are made on speculation right now, rather than based on true value and potential for making money.

But if that doesn't come true, and everyone realizes this, then there will be a crash. Valuations will fall, and the initial investors won't be able to make their money back because those photo apps weren't making any revenue, let alone profits.