all alts have pairing such as btc,etc,usd,usdt. as of now the btc pairing is the most widely used, we could see the alts go down when btc go down because that altcoin translates to how many satoshi and then only derive the usd value. If you trade in an exchange and find that btc is moving slowly. When you sell you see the pairing for btc vs the pairing for eth , at times maybe you see more profit in btc than in eth (as said because btc is rising slower) thus most likely you sell in the btc pair. For this I think is one possibility that btc may gain when its price is moving slower. In fact, gaining slowly , constantly. what do you think? this is just one of my thoughts, probably a good one to hear feedback.