It would be speculation for anyone here to say what the reason for the bulge is. It's likely a bowing out from settlement, as you say. Is it possible for any water to have entered in the past and caused the wall to sag? Is the plaster hollow sounding when you knock on it?

I had a bulge and spalling at the top of my plaster walled stairway at the ceiling. It seemed that the cause was that a crack developed in the plaster and with the moisture in the house migrating in and through the crack was exposed to the cold in the attic (through conductance in the framing and lack of sealing) and gradually weakened the plaster and it swelled and bowed. I just chipped out the plaster to a solid base and capped it with overlapped fiberglass mesh and Durabond 90.

You could drill a small hole and prode about to determine what you have. I gather it's your intention to eliminate the bulge before you paint regardless of how big the problem is.

If your case is from settlement, you may end up with a large hole by the time you remove the bulge. I posted a message on the thread on this page that has to do with repair of plaster walls. I referred to a Ask This Old House episode in which Tom did a repair of a large hole in plaster wall.

Ted

I read in a book once that you could drill holes into the plaster, squirt some liquid nails type adhesive in behind the plaster, and then press it back onto the lath with a bit of plywood braced up with some 2 by fours. I don't know if it would work well, or if it is too large an area...the plaster night just crack and fall off of the wall....
It might work though...
Good luck!

could be some old electric buried - that's what some of our bumps were - yikes live wires buried w/masking tape at the ends

Depending on the size of the bulge, another possibility is that there is a gas light pipe there, and was just covered over when the fixture was removed...sounds like a stairway is an ideal place for such a thing.

My house was built in 1907, and I have come across several buried pipes for fixtures--mine were originally combination gas/electric. Luckily I still have one of the original fixtures in the attic, though the gas is cut off, the electrical socket still works fine.

I'll also mention that every upstairs room has a capped off pipe in the floor where heating fixtures were originally attached! I have left these in place as part of the historic charm!

Today I looked at house built in 1892..thinking of purchasing it - or at least proceeding with an inspection. There were several bumps in the plaster walls. In several locations - you could tell someone tried to patch the walls but did a a horrible job.. Is there a "safe" # of bumps in a house... meaning if there are 2 - is that better than 10? Would an increased number of bumps or damaged areas indicate excessive house settling? Which could then mean foundation problems? I looked at the walls in the basement and did not see any bowing. I did notice in one area that the brick had been replaced with cinder block and Im assuming cinder block didnt exist in 1892....?