The answer to 'why is my retaining wall yellowing' is that it is likely due to mineral deposits or efflorescence. Efflorescence is a white, powdery substance that can form on the surface of concrete or masonry, and it is caused by the evaporation of water that contains dissolved salts. Over time, these salts can build up and create a yellow or white discoloration on the wall.
The yellowing can also be caused by mineral deposits from the soil or water seeping through the wall. This is common with retaining walls that are in contact with the ground. The minerals in the soil can leach into the wall and cause discoloration.