I'm not familiar with this effect. It seems to be getting worse by the day. Is there a common cause for something like this?
This phenomenon may be explained by the presence of moles in your garden. I am no expert on the lives of moles, but I have a similar situation around my house. They seem to prefer to dig soil up around fixed structures, such as walls, for some reason. When I first moved here, I would dig the soil back with a shovel or water the raised sections every time I see new activity, and eventually they appear to have given up and diverted their tunnels elsewhere.