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.
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.