" We did just have the dormers painted (with old paint scraped off and repairs done, etc.). "

Exterior painted? Not sure how but would say it must be related. You sure that is all they did?

Yes, exterior painted. I think they may have replaced some boards.

When they replaced boards, they used nails, did any of those nails come through into the plaster. Looks like it in pictures.

The painters came over and looked at the holes and, although they too were mystified, agreed they must have in some way been responsible. They are taking care of it. Thanks, folks!

If they were tugging on the walls, you can easily get drywall nail pops like that.