A 200 and 21 weekly MA crossover are never happened in bitcoin history ever. Such moving average crossover almost occurred back in September 2015 but the price immediately reverse direction to the upside so that the death cross didn't happen.
This time the price of bitcoin has stayed below the 200 weekly moving average for a relatively long period of time. Bitcoin tried to break above the 200 weekly MA but got rejected which increased the potential downside for bitcoin and increased the likeliness of a death cross to happen.