Why do I instantly get demoted?

I get to a superior division than mine, and Ive been pretty much always wining my next 3 games after the promotion, then I start going down, If i get to zero points though, I instantly get demoted..... Ive read that you can have 5 losses being at zero points. But I never get that, If I get to zero I instantly demote. Is it this way always?

I asked this because like 4 months ago I remember that I never won my games after promotion, but that even though i had losses being at zero points i was still in that division.