any idea of the cause / repair of these wall "bumps"

Water is causing it

Pop one of them and see if there's water inside.

My first thought would be water intrusion, too. But the pattern looks almost like something caustic was sprayed or splattered on the wall. Drain cleaner, perhaps? Shave cream? Toothpaste or hair products?

What's directly below where the spots are? Sink or toilet/sprayer maybe?

Just to make sure, you HAVE tried to wipe it off and it won't go away, right?

The glue in the wallpaper is reacting with the wallboard/plywood glues/formaldahyde, etc. I believe those are a chemical salt crystal forming at the interface of the two materials. It's much the same as treated would reacting with aluminum to form a corrosion product.

GMandJM wrote:
My first thought would be water intrusion, too. But the pattern looks almost like something caustic was sprayed or splattered on the wall. Drain cleaner, perhaps? Shave cream? Toothpaste or hair products?

What's directly below where the spots are? Sink or toilet/sprayer maybe?

Just to make sure, you HAVE tried to wipe it off and it won't go away, right?

If it was from a surface applied product, there should be some discoloration or surface distress. I believe the affect comes from the backside of the wall paper.

Hard to tell from the photos. I was thinking that the lighter/whiter spots might have been where "something" got onto the surface and then penetrated from there and leached outward under the surface.

No matter what it is, I admire the OP's restraint. I likely could not resist popping one of those puppies open to see what's inside. :B

-j

Those bumps are mold growing behind the wallpaper.

Been there, done that.

Denny and Cindy Henderson
fulltiming, currently at Jellystone Park at Natural Bridge Station, VA

Mines had a similar pattern above the shower. No mold in my case was a crystalline reaction with the glue used to adhised the vinyl to the ceiling aluminum air duct. Sanded it, spread it over with vinyl grout then painted and looks great after three years.

Did you find out what it was? I have the same pimples underneath a window.

We had similar over shower from skylight leak. Leak fixed and bumps did not spread any. I need to sand/paint as Cousin Eddie93 states.