It is actually very simple, most of the altcoins are mostly traded with BTC pair, so there actual price stays in satoshis and not any FIAT value.
Now, suppose an ALT is worth 500 satoshis while bitcoin price is USD 10K
After few months, the ALT is still worth 500 satoshis, whereas the bitcoin price comes down to half, i.e USD 5K

This will make the ALT price is FIAT come down by half as well, as even if its still 500 satoshis but the satoshi value is now half then what it was previously.
I guess, there is no way to simplify this.