My wife and I bought our home 3 years ago. It is roughly 50 years old, and had recently been shored prior to our purchasing it. There were a few cracks in drywall here and there, nothing that wasn't easily patched and painted. Six months ago, we had spray foam insulation installed in our attic, and since that time, I've noticed some cracks starting to appear once again, some in places I'd previously patched, some new. As I was inspecting a re-opened crack, I looked around the rest of the house, and found an interior wall where the seam where the all meets the ceiling has split, almost the whole way across the room. I'm wondering, what could cause that? Could it be related to the weight difference caused by the newly installed insulation in the attic? Could it be some natural settlement following the recent shoring job? We also had a drought for most of October through present, which could also cause some settlement issues. I have visually inspected the slab, but I don't see where any significant cracks have appeared. Has anyone seen anything like that before, or had a similar experience?
You indicate that you have been in a drought since October. Combine cooler weather, better insulated roof, and heat being on; the structural framing of the house may have shrunk.
I experience some "seasonal" cracking and shrinkage in my home. See what it looks like in July/August, if the condition persists, consult a local experienced contractor.
If the roof is structured with pre-built trusses, then this site, http://inspectapedia.com/interiors/Roof_Truss_Uplift.php could explain the condition you observe.
Briefly, if different parts of a wooden truss are subject to different temperature or humidity, the changes in individual dimensions can cause the truss to arch in the middle, and draw away from walls below.
Cracks where the walls meet the ceiling are quite common and can be caused by a number of things including thermal expansion of the timbers, moisture and even vibrations caused by normal occupancy.
Consider the seam between two sheets of drywall in-plane on the wall. Expansion of the structure is not likely to split this seam because the timbers expand relatively uniform and elongate in the same direction.
Now, between the wall and the ceiling you have ceiling timbers expanding laterally and the wall timbers expanding vertically. Though they are fastened you will have small strains focused on the corners.
Drywall mud is not a very elastic substance when dry. To address this problem you should run a bead of paintable caulking the entire length of where your walls and ceiling meet. You will still have the wear on the corner but the caulking will now show a crack like the raw mud will. Alternatively you can use crown molding to obscure the corners.