What exactly do you mean by "backed"? If we look from your message, I think your question is more or less why altcoin took a big hit when Bitcoin dump. That was answered by some people above.
In crypto, people usually use the "backed" term to indicate that the token/coin has other assets to guarantee its value. There are some altcoins backed by BTC such as WBTC (or at least they claim to be), but not all of them do this.
Yeah, I guess I'm misunderstood what "backed" means. Answering the question from the OP, I think it's because bitcoin is the fundamentals of crypto as a whole, so where there is a change that occurs in bitcoin, other cryptos will follow it (although this doesn't apply to all existing cryptos, but most and more 80%).