Well, what is certain is that when the price of Bitcoin falls, it will be followed by a fall in the price of Bitcoin, but the question is why prices tend to decline when the price of Bitcoin rises all because the dominance given by altcoin is still small so the trading volume is also small. it happens at altcoin too, but if the dominance of Bitcoin rises quickly then the altcoin price can be sure to place ATH or the bull season arrived.
Bitcoin is about to reach the threshold of slowing down and falling and that time will be the time when capital flow will flows into Altcoin
This hasn't been the case for recent falls. When the price of bitcoin falls, people prefer to hold that money in fiat equivalent than a crypto so they could buy back bitcoin when it reaches the bottom. Thus they starts buying stablecoins. Though we may consider stablecoin as an altcoin but they have no increase and value and hence bitcoin's dominance keeps on rising.