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 that's a very good explanation so probably that's how it's working for now.
Even more harder question now... When it ends? :-)
I'd like to share my point of view on this.
Bitcoin probably will again grow to 6000 sooner or later. Maybe it will hit the new record about 6500 - who knows. During that Bitcoin might need money which will arrive from new investors and some of it from alts (which might go down even more). But then it will be time for correction on Bitcoin. Maybe massive 30-40-50% correction like before.
So here is the last question: how deep will sink alts like Ethereum, Litecoin or Monero? 30-50% from current level would be serious for them.
I'm honestly a bit worried guys that some bad sh@t is happening to alts as none of them became REAL currency and instead of new heights we might not see new records for maybe long months and who knows what will be later.
Just a quick thought wanted to share. I might be VERY wrong of course.
Thanks!