The cause is mostly because of people's perception or as most them like to call it: FUD (Fear Uncertainty Doubt)
Right now, over half of the people who went into crypto business doesn't fully understand how it works. To top it all of, the majority of them "only" know about Bitcoin.
So when something happens to Bitcoin (like when the government or media spreads false rumor about it, or when an exchange gets hack or something), It is expected that other alt coins will follow because it will trickle-down to the lower alt coins