People need to realize that all coins today are inherently linked to btc.
Most people only trade alts to try and get more btc. In a falling market, your goal switches from maximizing your returns to instead trying to best preserve your capital as a result everyone will flock to those coins that they feel have most value or are most reliable from a long term point of view. For crypto markets today that is bitcoin, as a result most people today are selling their alts and converting it into bitcoins. Now this should argubally create buying pressure on btc, however the price of btc is falling as well which indicates people are converting to btc and then quickly exiting back into fiat