It's not a new thing, altcoins are more volatile than bitcoin, therefore that fluctuation you witnessed is just normal.
We may see that altcoins is stable for some time, but that doesn't mean the market has already become stable as anytime price could significantly move, either it will pump or it will dump, that's the reality, so better get used to it.
If we are talking about volatility mate, there's nothing more volatile than bitcoin, you've just said it coz you see a huge gains on some alternative coins, higher than what the bitcoin had last week but that does not mean that they are more volatile than bitcoin. As you have said, altcoins are more stable for some time which is not a true definition of volatility. Volatility is defined by wide price speculation and heavy trading which makes up the price, in the case of atlcoins the rise are just pumps, a long time needed pump for the coin not to die. Seeing the market now, It's all good, bitcoin is rising and altcoins too, secure what you need to secure.
Bitcoin though still at a good price, but it's -1.57% today.
Bitcoin is looking good, it is still up 20% from the past week, as for my speculation to it, it will stay at $11k for quite some time and pump again by mid September