You wouldn't know because you are just new to the space, if you were an old experienced Bitcoiner, you would understand by now that Bitcoin is often referred as the father of all crypto because during bull season for Bitcoin, as the price surges, some other projects uses that avenue to pump and dump their tokens.
Yes, he need to research for more to find the answer. Bitcoin is the leader of cryptocurrency so altcoin will follow behind Bitcoin. When Bitcoin increase, altcoin follow increases but when Bitcoin is down, altcoin following to go down.
But when Bitcoin is down, many altcoin can not resist their price to go down a lot and not easy for them to increase back. Maybe only some small altcoin list that can increase but that will need time before that is happen.
I have also been like OP before when I usually wonder about the reason why altcoins normally fall when Bitcoin price dip too but know I know better, for a coin that stands out like Bitcoin which have the largest market cap among other coins, what else do we expect? Most investors have that panic sell to start selling their coins when they see Bitcoin falling because they know that it will also affect the price of those altcoins.