Most people believe that the price of altcoins' is determined by the price of bitcoin and that they tend to follow it's market movements.

I partially disagree with this. I have studied prices of coins and tokens on CMC and the fluctuations of each currency seems to be unique and independent of any other digital currency. Some show strong pumps when BTC goes down and some heavy dumps when BTC is rising.

Undoubtedly, there are market forces and factors which affects most currencies, this is not influenced by bitcoins price, it rather controls bitcoins' price as well as others.
So when trading or investing, make sure you treat each currency differently as they have different utilities.