New plaster peeling with lining paper

I recently skimmed the walls in my bedroom and the other day started to line one wall where I was going to put a radiator (so that if it went wrong the radiator would cover it - as it was my first time at hanging paper!).
I dont have any skirting boards yet and thought that I could add them afterwards so havent trimmed the bottom of the paper. I had a look today as I was going to finish the wall and at the bottom, the paper had pulled some of the plaster clean off the plasterboard!

Is there any reason why this would happen? I didnt prime the plasterboard before plastering but didnt think I needed to. Should I get Skirting and architrave before lining the walls? I have primed one wall with watered down wallpaper paste abotu a week ago - do I need to redo this when I come to lining the rest?

You should always seal new plasterboard, and the best way prior to hanging paper is to use an oil base thinned down undercoat, this will make life sooooo much easier if and when you have to strip any paper from plasterboard.

I would carry on sizing the the rest of the walls prior to hanging L/P, just knock up a thin solution of wall paper paste, and whack that on all the walls the night before your due to start lining.

Dont forget to let each length of L/P soak for approx 5-8 mins (till supple) before you actually hang.