What’s going on here is that altcoins trade predominantly against BTC (i.e. these are crypto-to-crypto exchanges), and BTC trades predominantly against the dollar. So if bitcoin rises in value, in order to maintain the same USD value, altcoins need to drop in terms of BTC. Traders talk of altcoins being ‘pegged’ to bitcoin, in the sense that a move for bitcoin will necessarily entail a move for altcoins. If more altcoins traded directly against the dollar rather than BTC, it might be different, but for now, BTC remains the major on-ramp for altcoins.