When BTC dumps, all other coins also dump, it is worst thing because even if bitcoin dumps, then it should not effect the other cryptocurrency price value in BTC. i,e when BTC dumps the price value of altcoins is also drops while it should increase because btc downs. When people will realize it that it is not in their benefit?
Do you also ask yourself why does altcoins dump also when bitcoins pumps? 
The thing is this its not every altcoins that falls when Bitcoin dumps, but majority of them do and its also not every altcoins that falls when Bitcoin pumps.
Explaining this is a difficult thing to do, but lets say that altcoins are mangoes and Bitcoin is the USD, there are times USD will gain value and mangoes will still be sold for the same price as they were being sold before
. Now that means that you now have extra USD and you can buy same number of mangoes and now have extra change remaining. Something like that I guess. It's because of the crytpocurrency marketcap as a whole. Whenever the marketcap is higher, there is more money to be invest in not only Bitcoin, but also the other altcoins. And, whenever the marketcap is down, there is less money in the market for people to invest in everything as a whole. You will have one or two coins that will hold their price regardless, but I doubt they would in a long term downtrend as there is simply not as much money coming in to the market to move the prices.