The biggest and main reason that seems to me is that people are afraid and people sell Bitcoin in need of cash. Since cryptocurrency or Bitcoin is a risky thing, they sell it.
When a war starts, people look for a safe side. Bitcoin is still a risky asset in the eyes of many. So when a war starts, people sell Bitcoin and buy dollars or gold. In such a situation, uncertainty in the market increases, then many people or big institutions want to withdraw money. So they sell Bitcoin. Another reason is that when there is a war, the stock market also falls, and Bitcoin often fluctuates with the stock market. So when the stock market falls, Bitcoin also often falls, meaning everyone presses sell options together and the price of Bitcoin rapidly falls.