They compare the altcoin price only to the bitcoin price, the $ price is irrelevant for them. Most of them got large quantities of BTC when it was still cheap.
So BTC goes up --> they have to sell all of their altcoins in order not to make a loss on their BTC number. Because for them if BTC goes up and alts stay the same/go down they make a loss in BTC.
BTC goes down --> they know that altcoins go down even more when BTC goes down (I do not understand exactly why it is like this). So they sell their altcoins for BTC and wait until BTC stabilizes and altcoins are at the lowest point, then buy altcoin and like this get more Altcoin than they previously had because of the altcoin discount.
BTC is stable --> they still seek to increase the number of their BTC and try to chose altcoins that have a bigger growth than BTC.
All those stupid forks just increase this problem, but as long as people profit from it, there will be many more forks since it's basically free money. It's one of the many ways to scam people, just like those massive amount of airdrop coins or some greedy ICOs (not alls ICOs).
This behaviour is also the reason why the altcoin prices will always depend on the BTC price (among other things, like for example that you can buy a lot of alts only with BTC).It will always stay like this because this is how you make the most out of your Bitcoin/Money. People also seem to follow traders blindly, contributing to this problem. I wonder what it takes to make altcoins price development more independent from BTC/traders. Maybe being able to buy Altoins with fiat would help. Or if people would wake up and realise that Bitcoin is already an old technology and there are way better options within the world of blockchain.