It is repeatable on this world it seems. I move westward past one of my walls and it seems to quite consistently happen. If it is helpful I put some markers in the area that it crashes when explored, past the furthest west wall at the top. https://www.dropbox.com/s/qhpnr6v9bkni7mf/Crashing%20Save%20...

Was exploring new areas of the map while artillery was clearing out nests.
Went back and it happens even without the artillery firing in the newly explored areas.