Thank you.
The correct answer would be,
Every alt coin is tied with bitcoin or compared with btc e.g eth/btc, xyz/btc because it is traded with bitcoin (you buy the alt coin using bitcoin or sell your alt coin for bitcoin). So technically every alt coin price is depending on the price of bitcoin. Well totally depending on the price of bitcoin.
Every time the price of bitcoin going up, the alt coin investor wants a piece of that cake for a short term therefore they would sell their alt coin holdings to get into bitcoin. You may ask why? It is because when the price of bitcoin goes up, almost all alt coins will go down due to the selling off. So the opportunity to getting more alt coins per bitcoin is more. This is one of the best way of accumulating alt coin holdings. Due to this, investors who likes to accumulate their alt coin will take position in bitcoin and wait for the price of the alt coin to drop lower. When this happens, the investors will then sell their bitcoin to get into alt coin again. Successfully increasing their alt coin numbers.
Now to the question of why alt coin price go lower when bitcoin price go low. The same explaination as per the above. Since all the alt coins are tied with bitcoin, the price of alts will also drop when the price of btc goes down. This is normally a temporary drop, when investors cashing out of bitcoin to get into alt coin. The delay between sell of of bitcoin to transferring to other broker, or position of buying into alt coin is when the price of alt is at its lowest. The alt coin price will only increase with demand or with more buys.
I hope this explains to you.