Basically the altcoins are mainly following the price of Bitcoin.
Bitcoin is the biggest of coins, the first of coins and the most known of coins.
Crowd's mentality is like this "ok, so I put my money into NEO, I think it's a wonderful project".
(the Bitcoin price turns red)
Crowd thinks "OH MY GOD, the crypto is crashing!!!! What am I gonna do???" ... (sold everything) ... "Fuuh, such a relief, I could have lost a lot more"
(crypto turns green)
"OH MY GOD, the crypto is rising again!!!! What am I gonna do???" ... (FOMO) ... "yeah, this is how it is done"

BTC as indicator