I think the main reason is that, people are pulling their assets from Altcoins and dumping it to Bitcoin as it is increasing. When Bitcoin starts to drop, we might see people who will pull out from bitcoin and dump their assets to alts.
This is not the exact reason for it.When bitcoin starts to get a price pump the trust on the whole cryptocurrency is increasing and thats the beginning of alts to increase their prices too, It is not becuase they dump bitcoin and go to alts instead.
