"Dominance" is an arbitrary thing too look at since it is just market cap ratio (bitcoin versus unlimited number of altcoins with unlimited supplies). It  doesn't really show anything. For example there could be a period where the scammers create a large number of shitcoins hence inflating the sum of all altcoin market caps which would then decrease the market cap ratio (falsely called bitcoin dominance) but that does not mean altcoins are being pumped.

The actual trend is that each time bitcoin price goes up or down, the altcoins get dumped.
But each time we have a bitcoin-bull-season where "fresh money" is coming in, the altcoins can get access to new wave of "newbies" who would buy their garbage hence the altcoins also experience the altcoin pumping season.