this time I can't understand where is the bad news behind this big dump

probably some big whale just empty their big wallet?
who knows but right now from a bigger value over 19k on coinbase we are touching a strong defeat

in 2018
Don't watch out for news. Not everything is news related.
It's just the market doing it's thing. We are in a bear market and therefore the bears are strong. Bulls have no confidence or any will to buy here, that's why you see these weak bounces and a more declining price.
Bitcoin's fundamentals are great and had never been better. It's the sentiment and the other side of the medal what is driving the market down. Normal behaviour.
So do not worry. What goes up, must come down, and what is down will go up.