The most intuitive reason for the decline in the altcoin currency may be the decline in bitcoin, because Bitcoin has fallen all the way, and all the altcoins can't escape the fate of falling.
On the other hand, the reason may be that people are not very optimistic about the altcoin, no one will invest, once the market does not need the altcoin, it will fall.
Altcoins really well repeat the movement of bitcoin prices, this is due to the fact that bitcoin has too much influence on the market. But sometimes it's not as simple as it seems. Sometimes the growth and fall in prices for altcoins do not depend on bitcoin, usually this occurs before significant events in the market.