That's the packed earth, a significant portion of your structural support and counterweight relies on compacted soil behaving like...soil, not water.
The thing is your stone wall is resting on soil or clay in most places you can build a dam, so as the water burden increases, such as with this freakish level of torrential rain, you simultaneously increase the pressure exerted by water load, erosion from water flow(especially overflow) and decrease the strength of the anchor holding the walls in place.
Which basically means the wall starts uh...sliding. Which is not something you can afford it to do for very long because the wall is much less strong once it's not in the spot it's built in.
Obviously, people will overengineer a dam to some extent, but previous to developing the discipline of engineering itself, and faced with a load that's completely outside of the designer's expectations, it's one of the natural flaws of dams.