If you have coin base pro it lists the 15 or so most popular coins with little graphs and +/- in a list. The whole list with the exception of one coin will all be in the green or the red, and the price charts are very close to each other. What links them all together? Especially if a coin is etherium based another BTC and another based on some other coin. The stock market does this kind of but not to the T like BTC and Alts.
That is because of :
Trading Not only that, many of the alts can't be purchased just like that , they are supposed to be traded against BTC which means that people will have to rely on bitcoins for them.
The transaction fee of these alts is also low which does mean that keeping them and transferring the money in them would be beneficial.
So when the price rises :
People does not only buy these alts to trade at the right time but as a whole the crypto community gets active and these lesser active alts gets a little boost from the peopleAnother reason !!
Much like ETH some of them also have a mutualistic relationship with BTC . Which means instead of competition there is healthy support!
At the same time the more bitcoins rise in price , the more media coverage the crypto market gets now people tend to believe that they have lost the bull therefore they try and find other options to secure a good wallet in the future.