The idea that bitcoin rising leads to altcoins falling is an unproven hypothesis, it is something that traders have noticed, but due to the various flaws in human observation, we cannot say for certain it is true.
There is one connection, when bitcoin is rising that's mean money is being pumped into it, money comes from alts, people are cashing from alts and they are buying back bitcoin. Money has its flow and its normally that market is affected by traders decisions, I mean masses. Today everyone wish to sell on top and buy back in bottom, how can market react on that money flow? With fluctuations of course.
Bitcoin has impact on alts price, when bitcoin rise prices of alts in fiat also rise. Bitcoin is the main mean of exchange for many alts and tokens.