What is causing these cracks under the shelves

[ruggiero]

I noticed these cracks under the shelves in my storage room. It’s an old building in Scotland.

If you touch it’s a bit crumbly, but the wall is quite solid.

The shelves are quite sturdy.

Any idea what is causing it and how to fix it or clean it up?

 

[RrogerD]

I wouldn't worry about it if it is under the shelves, you won't see it anyway.
If you're really concerned then fill with decorator's caulk. It's flexible. It's the expansion and contraction of the wooden shelves that have caused the cracking and the plaster to break away.