Basically the same thing that cause the Wall Street crash. Bitcoins value isn't really tied to anything. You can't buy anything with it. The only reason it has worth is because people want it. If everyone sells at once the price goes down. This panics everyone else who also sell all their bitcoins. Value goes down
Thereby the real answer is anything that would tempt or cause people to sell it at once. economy tanks and people need money? They'll sell their bitcoins. Bitcoin spends a little too long going steadily down? people will sell. And once people start selling the price will drop, meaning even more people sell. But the horrific part is people are buying bitcoin as an investment rather than an alternative form of payment.
What this means is that people will only keep buying bitcoin as long as its value keeps rising and they think they can sell for higher. But once the increasing valuation will slow, there will be a massive sale of bitcoin because people will try to move their money. The price will drop drastically and the people who haven't moved their money will lose basically their investment.
Still the reason since it's been treated as investment form of assets and not being used to serve it's purpose. Better not to be affected of any cause
of the this market crashed if you do invest to gain profits then wait till you achieved such goals. Forget being negative as you will keep losing your money while you are inside this investment venue.
Bitcoin still on a speculative time, there's still few who adopts the system but who knows what will happen after another 5-10 years. Those collapse
on it's value cause by compound selling pressures both from whales and weak holders.
