One reason why they also rise is that their price is based on BTC. If BTC goes up, these altcoins also go up even when not bought by volume.
I always try to avoid the forks of the major cryptocurrencies unless they are forks that are well known and have survived years with different market conditions as major crypto. Bitcoin has over 70 forks and there have been some cases of newbies trying to invest in BTC but due to one reason or the other, they end up buying a worthless BTC fork because they thought it was cheaper.
You can't avoid them when the coin you hold are being forked but since they are free money, take it because you earned them. Many of the BTC holders from the beginning claimed their BCH and BSV for their free money.