I don't think all altcoins depend on BTC, the proof is that there is altcoins that can still survived if BTC goes down, but most altcoins follow the btc trend, maybe because BTC is so dominant in the crypto that many altcoin holders sell all their assets to buy BTC,
but few altcoins do that. mostly if there is a decrease in bitcoin, there will be a lot of altcoins that go down, especially if the altcoin is just a momentary fomo action, of course it will come back down also following bitcoin.
Simply because bitcoin dominance still high, if there's some coins who can move against bitcoin
trend is some because of the update and most are being used for pump and dump. Be very careful
if you are playing with the market, this kind of action mostly the reason why many people got stuck
and lose their money.