Terry Pinnell wrote:

I'm puzzled how it got to this state, as I'd assume the builder laid
a foundation under ALL the bricks, including these at the edges.
Maybe years of gardening has been a factor...
I should think it's either just compacted grit sand under the blocks, or
was dry laid grit sand plus cement, that's now hardened.

but if you haven't got proper concreted-in edge course, you're going to
have problems from the blocks walking.