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More of the altcoins are bought by exchanging bitcoin. Let's assume bitcoin as a piece of bread costing 12$ and you could buy any groceries in the market by exchanging bread. Suddenly if bread price went to 6$ then the amount of groceries you get would alter(decrease). Similar happens with the uptrend.
** Most importantly decrease in btc price might not make alts costlier the reverse also may happen because many people start dumping their btc to some alts and the supply of alts increases as a result it may become cheaper, similar happens when btc goes up.

Bitcoin is the founder currency and holds the majority shares in the crypto world. Market dominance of bitcoin is around 32-35% of total crypto currency volume.
There are about 1900 plus crypto currencies and no other coin has such a huge dominance. Therefore, all the crypto market follows the Bitcoin.
Some of the coins are getting trust of the investors are these coins. With increases dominance of other coins in future we may expect that crypto market will not be dependent on one single coin but for this we have to wait for a long time.

because bitcoin is a mother currency and all other coins are dealt with btc while trading, as someone wants to buy a coin they will buy btc and then that coin, and also when an ico comes the company takes your btc and gives you token and if the value of btc falls , then the amount of your investment company have to run its business also falls and with that the altcoin value also falls.

The cause is mostly because of people's perception or as most them like to call it: FUD (Fear Uncertainty Doubt)

Right now, over half of the people who went into crypto business doesn't fully understand how it works. To top it all of, the majority of them "only" know about Bitcoin.

So when something happens to Bitcoin (like when the government or media spreads false rumor about it, or when an exchange gets hack or something), It is expected that other alt coins will follow because it will trickle-down to the lower alt coins

Point 1: Most of the currencies are bought as BTC pairs so BTC goes up they go up, BTC goes down they go down.

Point 2: FUD, fear uncertainty doubt, as no one is sure where a downtrend is going to stop people start selling to buy again at lower price which effects the price of a coin more than the drop of the BTC percent.

Bitcoin is the major cryptocurrency and that is why most cryptocurrencies are exchanged with

it,and since most cryptocurrencies are trades with it then it will be alright expected that whatever

happens to bitcoin will also affect the other cryptocurrency so that is why whenever the

bitcoin price goes up then the value of other cryptocurrencies goes up too and when bitcoin

price goes down then the value of ofther cryptocurrencies goes down too...

Bitcoin is the mother currency of other cryptocurrency at first only bitcoin was the coin in this world at time of 2009 then after other cryptocurrency started coming in the platform under bitcoin so the coins are on bitcoin coding so this is the main reason after bitcoin price and other altcoin price