Hi there,

It could be a few reason causing hairline crack. First is the painting works, if skim coat isn't applied well or it hasn't completely dry before painting will cause this. Secondly, it might cause by the rushy brick work. Thirdly, which is the worse case, the foundation of the house isn't firm enough and sinking. But from the depth of the cracks, I think is not because of sinking, probably just the unprofessional works completed. But it can be fix no big deal