this could be a factor of all three. An improper connection, a dropping foundation in one area, stress somewhere is causing this to open up.
You can usually investigate these cracks and they will tell you who is pulling who down.
Here is a test, does the crack close up during the winter or rainy seasons? Does the crack open up in the summer and dry times? If so on both, then its the foundation.
If its relatively stable, then it could be just improper add on connection.
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