I think one of the causes is the effect of trade war between China and America
and also the global economy is weak
I disagree if it is said that the decline in the price of bitcoin is due to trade war between U.S with China. A trade war has been going on for almost two years. And I don't see any connection between trade war and the current decline in the price of bitcoin. Bitcoin prices are moving lower due to the absence of positive sentiment that lifted prices.
A routine cycles coming from the whales who have a broader knowledge regarding to this business. It's not related to any trade wars as you mentioned that this issue was been there for almost two years. The relevance of this decline from the whales movements are much closer to possibilities rather than the trade war issue. Big holders after shaking the market will start to buy back and lift the value to rise again without any notice from spectators. hold tight and wait for the time of the bull.