How can the prices of 100s of cryptos all move in the same direction at the same time?
Because traders more buy at the same time irrespective of the coin, because traders sell at the same time irrespective of the coin also.
For holding, after bitcoin halving, people fomo and holders buy more during the periods and sell more during the bear time periods.
The price is mostly controlled by bitcoin price, do not get me wrong, I mean when people are buying bitcoin, they are buying altcoins too, when they are selling bitcoin, they are selling altcoins too. That is crypto market and that is the reason.
Mostly because bitcoin (BTC) still acts as sort of the "index" of the cryptocurrency space (just like the SPY with stocks), and that BTC trading pairs have the most trading volume and market liquidity.
This is how it will always be, that is how cryptocurrencies are, following the same trends, but percentage rise or fall always differs.