Usually when BTC rises, the alts rise more, and as a rule, when BTC fall, the alts devaluate too when compared to BTC,
If you think prices will rise, then you should keep some alts just in case and do less diversification to keep it manageable, if you think prices will fall, the best is sell everything to BTC then sell the BTC and wait
This has been my philosophy. I can only speculate it is because people feel BTC is overvalued and go with alts at peaks. But whatever the reason I usually follow this.