When bitcoin rises, the altcoins keep the price and if when the bitcoin falls the altcoins fall even more. Why do altcoins usually follow bitcoin?
There is no definite answer to this, but this can be due to a number of assumptions made correctly.
Bitcoin functions as an index of market psychology and users follow what movements it makes to determine future trends. So when bitcoin is dropping in price, the people who had bought with lack of understanding and faith in the altcoins will sell because they think this is the end of that altcoin.
Those who sell altcoins may not be that much capable to understand that if they hold they might actually get a price to break even and exit with profit, therefore a lack of patience.
Because this market is unregulated and panic spreads quickly, it is difficult to control a downtrend quickly. But the opposite also becomes true, when it goes up all coins start going up as well.