Bitcoin is falling because the alts did not follow the recent price surge.
In a real bull market the two markets alternate ups and downs. Thats how bitcoin can sustain an increase in its value.
That's what happens in a
bubble. We're not there yet. It's a good thing too, because a vertical move straight to a new ATH this summer would have crashed pretty quickly. Building up support with multi-month corrections is the key to building a major parabolic move like 2017.
This one, it takes months and months building support and I still do think that we are still farther away from the parabolic rise of 2017, heck we might not end get to $20k this year as others may have predicted.
I wouldn't say anything happened because of alts. Alts are mainly just correlated to the BTC market. The primary explanation is just that BTC is still in correction mode. Supply and demand and all that.
I agree that alts needs time to grow so that the market will be healthy as there are correlation between BTC and alts. But how can alts grow when altcoins investors are not willing to gamble on them and would rather go into BTC. Altcoin market of 2016-2017 is very different now as investors are getting smarter.