Great question — answering as the UphillTech team:
What I see looks to be a normal crack (If one can call a crack normal). Due to settling and/or expansion and contraction of dissimilar materials. Often with the seasonal climate change these cracks happen. When I lived in the NE of the US this was a common wintertime thing. Especially in older homes. You may want to consider filling the cracks with acrylic painters caulk, rather than taping and mudding. The caulk being somewhat flexible can help keep the crack from arearing again.
Full answer by RMDman: https://diy.stackexchange.com/a/312284 (CC BY-SA 4.0).
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