the decline in the value of each altcoin that exists may be due to a decrease in someone's confidence about the future of the altcoin itself, or also because many scammers take advantage of the opportunity to find investors and steal their money, or also because investors are not interested in investing in them. old coins,
Now the cost is decreasing for the same reasons for which it increased - the change in the value of bitcoin. When Bitcoin starts to fall, traders, fearing for their profits, start selling.
If bitcoin falls, then the entire market falls - with such thoughts, traders begin to sell altcoins looking at the fall in bitcoin. This is a chain reaction leading to the results that we are now seeing.