This is happening when BTC is having a bull run. Altcoins' price is tanking because people wants to get into the BTC train. But, you must always think that this is not an absolute rule and this can be changed in any time at any type of market we currently have. Look at what happened last 1st and 2nd quarter of this year, BTC rallied hard but altcoins' price also increased giving traders tenfold profit in their portfolio. We can just speculate on what's gonna happen but the reality is that no one knows what will happen. There's no absolute theory or practice to it. That's why diversification is very important.