It's usually due to moisture. If you don't have a leaky roof or foundation problem, it may just be environmental conditions, in which case running a dehumidifier should help.
I agree with Alyssa and Sharon. If it is a leak, I also add that it could be a pipe in the wall, could be leaking around a window, could be a leak from something upstairs, if this is a downstairs wall. So many possibilities, and not enough info from you. ☺ Might not hurt to call a professional person. Good luck.
Moving into a home built in the 1980"s, when entire mirrored walls were quite "the thing!" However, the workmen currently upgrading ceiling, painting, etc., have said...
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It's usually due to moisture. If you don't have a leaky roof or foundation problem, it may just be environmental conditions, in which case running a dehumidifier should help.
Exposure to water is the reason .... you have a leak somewhere, fix first, then remove and dispose of affected drywall and replace with new.
I agree with Alyssa and Sharon. If it is a leak, I also add that it could be a pipe in the wall, could be leaking around a window, could be a leak from something upstairs, if this is a downstairs wall. So many possibilities, and not enough info from you. ☺ Might not hurt to call a professional person. Good luck.