Old investors are mainly diversifying into altcoins, new investors are more likely to only focus on bitcoin.
Most people seem to accept than an investment in an altcoin they like means that a bunch of other people will do the same later on and push its price up both in relation to btc and their fiat (if they're trying to grow it).
A high alt dominance is normally expected about this time. I think the only time I saw dominance go over 80% was when bitcoin bled down to $2.9k.