Bitcoin vs alts price action

I still cannot wrap my head around this concept:

If Bitcoin goes up alts go down
If Bitcoin goes down alts go down
If Bitcoin goes sideways alts go up

What is the reason for this? They are traded against each other, so if one goes up the other one should go down?

Yes its straight forward that when Bitcoin goes up in value alts are worth less and therefore ' go down '
But the opposite is not true for some reason.

Note: this has nothing to do with the market crash which is going on at…

Bitcoin vs alts price action