BTC is the king of crypto, when BTC moves, whole crypto market moves. Currently BTC is about 38% of total crypto so any change in BTC has alot of weight. it also has to do with the base currencies against which people trade. when the new money comes, it is mostly in BTC or ETH first before it is converted to Alts.

i have noticed 2 different types of movements in alts. if BTC goes up or down sharply the behavior is different then gradual decline or increase.

another factor is automated trading bots. Myself included now in that list of traders who use script or an automated program and they happen to follow similar pattern across the invested tokens. Bots are also the reason for unexplain sharp dips and spikes happening all over regularly.