Good question, i may be wrong and if so, someone will probably correct me and explain it better, but i believe it's like this :
BTC going up -> money goes into btc for a quick ride to skyville
BTC going down -> people selling btc into fiat for protection and also money diverging from alts into bitcoin because it's the messiah coin and every dip is much appreciated to bagholders
I think this is the argument I will agree with to a large extent because even me I have been looking for a provable explanation to what is actually going on and I think the reason why we always compare bitcoin price with alt prices is because we believe that they are perfect alternative of each other and their relationship should be inverse but we forget there is an alternative of the two which is fiat that can go into and out of bitcoin without touching alts. That alone as nulify any relationship we might want to establish between bitcoin and alt.