I see several reasons
New players come on Bitcoin market - institutional traders who had long planned to enter on short, because they did not see any fundamental strength or significance in Bitcoin as a currency or Bitcoin Network as a payment system.
80% of Bitcoin's turnover belongs to only 3 countries - South Korea, Japan and Vietnam(yes!) You must list countries to limit the turnover of funds in the country, and it is really problematic for people to buy an even dollar. They transfer their capital (dongs, yens, wons) to the cryptocurrency, in order to be able to not depend on the country's economy.
When rumors appeared a possible crypto-currency ban in South Korea, as well as on the new law, which obliges large miner pools to move out of China - the market reacted instantly.
Following the bitcoin, many altcoins dropped too and, of course, his forks. This phenomenon calls correlation. So this is nothing more than the actual dependencies of the main altcoins and forks from Bitcoin.