It is less risky only when the price grows. But what about when the market changes its trend from bull to bear? Like I said, passive investing is suitable only for Bitcoin. Buy it and forget about it for few months.
With altcoins it is different. We all have noticed that most of the time they copy price changes of Bitcoin, and from time to time show their own growth. It might turn out, that when you have invested in an altcoin $1000 and waited for few months, its value might now have changed. But, if you trade it when its price suddenly goes up, and be lucky to buy back when the price is low, your profit would be much greater. That is why I have said that trading should be done parallel passive investing. Or at least know basics of market trading to be able to catch "sell high buy low" moment.
Mostly it is indeed copying Bitcoin but might not be too direct copy. More like take it as investors would lean to other alts as well when they saw Bitcoin's price is quite seeing the green arrows lately. That's why it is common mindset for investors to go for alts once Bitcoin starts to show it. But on the bad side, people tend to dump alts when Bitcoin dumps as well that's why there aren't that many surviving coins.