I suspect that the shrubs and tree to the left of the picture had started destroying the retaining wall. You can see that there are wall-coloured encroachments onto the pavement on that side.

The house isn’t at a corner, as such - the street to the side is bollarded off.

So, given that the trees and retaining wall needed replacing, I’m guessing they took the opportunity to build the wall higher and level the whole garden. I probably would try to level it if I was rebuilding the wall too.

I wonder if not backfilling the wall with soil means it no longer counts as a retaining wall, and thus circumvents some planning / structural issues.