why are all these cryptocurrencies are going down?
One basic rule of trading, never buy when everyone is buying. If you did not enter a market when it is still on the huge lows, do not bother yourself much again about such a market when it starts pumping. However, a lot of correction is going on at the moment and you should learn to use your stop loss if you are trying to enter a market. Nevertheless, also understand that patience is a virtue and you can just chill or look for more money to buy in when they go lower, so you can leverage on some good profit when they get pumped back.
Its not a rule of thumb that whenever price of BTC goes up, cryptos will start turning red, what happen is its a market reaction that alt holders will start to cash in an enjoy the price surge of BTC but its not forever, it will recover just be patient and hodl. If you have extra and still willing to take the risk, buy more alts since the price is down.
Absolutely, nothing lasts forever. The going down of the alts are just going to happen for a while, very soon after the correction is over, most traders will start buying in back and then we see the prices going back up. Just like you said, instead of panicking, the best thing is to see these moments as an opportunity to make some profits from the dip.