Altcoins prices are affected by Bitcoin price movements may be due to several reasons
- Bitcoin is the most dominant coin in crypto market (approx 50% dominance now)
- almost all altcoins are traded in Bitcoin market with enough volume as compared to other markets
- volume of bitcoin is nearly around 30% of total volume which means most of the traded funds moving according to Bitcoin that's why whole market is being affected by Bitcoin movement
But this is not always true becoz it depends on market trends,TA, FA and many other things

Also there are many ERC- 20 tokens which have market only in ethereum and these tokens are affected too by ethereum movement

I prefer you to do not rely on one thing as movement of any coin depends on large no of factors
Don't trust blindly on something, doing own research is a good habit