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%).
Yeah, I understood the same from the "backed" as well but when you think about it, it can work the same way as well. Altcoins are sort of "backed" by bitcoin in that sense too because when bitcoin goes down the price of all others go down and that's backing, and when bitcoin goes up then price of altcoins go up as well.
I believe this dump caused the price of all other coins to drop as well because we are talking about a massive sell off from the people and that's understandable, I do not think that bitcoin started the dump and everything else dropped, I believe all crypto had a significant drop all together at the same time. This could be explained by the time difference, when there is a "backing" or anything like that, it has a time difference, bitcoin goes up first then a week later or a few days later the price of altcoins go up as well, this was all together at the same day even same minute.