Agh! Why is my paint splitting?

[Playduh]

Touching up a small area of wall, same paint as used originally. Sanded down and cleaned area before I painted. Now as it dries the paint is splitting length ways.

What have I done wrong?

You need to water the first coat well down, otherwise the walls suck the moisture from the paint and it cracks as it dries.

Did you wash the walls before painting to remove grease/dirt/grime?
Did you stir the paint before applying?

Washed walls down and left to dry.

Then stirred paint. Possibly not enough.

Do I sand it down, clean and try again with thinner, better stirred paint this time?