Ghideon

I am curious; did you find out what happened?

Guesswork from what we have so far: If there was an old wooden wall built before the current one, is it possible that the stone wall was build at exactly the same location? Since the wood under water may have been well preserved it was used as a foundation. Now, after hundreds of years, the old wood is decomposed and cannot support the stone wall. This caused the stone wall section to move almost straight down.

Was the wall built on top of parts of an older structure? An old wooden construction could possibly have supported the wall for a long time. Here is an example; oxygen-poor and humid environment slows down decomposition of organic material.

Is the water frozen in winter? (If the river is near your location in your profile then I guess not, according to a quick googling)