Look's like something is moveing (foundation/frameing). You need to have this looked at by a professional,there could be something far more serious than drywall cracks.

The stairs above it were replaced a few years ago, I wonder if it's related to that. Thanks

If one side of that drywall is nailed to bottom of stair stringers that might be the problem,not worth worrying about. If that's true I would install large crown molding fastened only to stringer to alow it sliding on wall as movement take's place. Bottom line,I can't tell you the facts unless you fly me in to look at it.

FWIW, I'll second what klem1 said, that's sound advice.