Usually we all want to see Bitcoin pric go higher and higher in each single minuet, But this is painful that when BTC grown and we drop out portfolio. Basically Alt market comes in crypto for multipurpose but most of use involved in alt market for making our bitcoins double, triple or more than this.
Here is everybody knew about that whenever bitcoin make sudden move weather its base on any news or due to whales artificial price manipulation this cause huge damage done in both our portfolio and alt coins too.
I think this is very strong reason of alt market crashes and many of new investors harms too from this behavior of market.
This is something you can not prevent. People hold altcoins to make profit. They main goal is "profit" not building a portfolio. When they see the price of bitcoin rising, they don't want to lose the opportunity of investing on bitcoin and make some quick profit. Thus they end up selling their altcoin and jumps into bitcoin investmnet causing the price of altcoins to drop, while the price of bitcoin starts to rise. They know that if they don't dump it, someone else will dump those coins, thus causing them to suffer from losses too.
Thus to avoid any loss and to maximize their profit, they invest on bitcoin.
Some people buys their sold coins back and cheaper rate.