The #1 enemy for subterranean walls is moisture. If you're building subterranean concrete walls in a humid environment like the Normandy coast, poorly engineered walls will quickly become mushroom farms, and they'll be constantly weeping with moisture.

These walls were built with forms. Ropes or another flexible conduit was used to provide channels for moisture to collect in. Presumably there would be an interior gutter that would drain the collected runoff away from the structure.

In contemporary construction, we'd use a selectively permeable moisture barrier, but this is a primitive approach to solving the same problem.

I believe that you are seeing spalling as a consequence of corrosion over time. As described by the General Services Administration, spalling constitutes "surface depressions that are larger and deeper than scaling, often linear when following the length of a rebar."

Spalling is:

• Also caused by pressure from under the surface of the concrete.

• Most often occur due to improperly constructed joints or the corrosion of rebar in the concrete

• Corrosion creates pressure as rust forms, which can push away large chunks of concrete, and expose the corroded metal below.

• Spalling that exposes corroded metal can be particularly problematic because the corrosion is likely to accelerate due to exposure to air and water.

Essentially just saltwater and shock doing its work on the rebar reinforcing the concrete.

Edit: Check responses below.

I didn't get an answer from anyone who really knew :(