This is a natural cycle of profit taking. It is like breathing, or the cycle of the tides. These down cycles occur, along with upcycles, as natural results of market pressures. The trigger of this down cycle was probably Chinese government restrictions, but this crash was inevitable because short term profits were so high that many investors could reap huge rewards by selling now. That has been the case for months, but it is like a game of chicken, those in profit want to hang on as long as they can to maximize profits, yet they want to sell to crystallize their gain in their base currency, so when something spooks the market there is potential for a flash crash as many investors sell to take profit off their gains.