Heavy silty soil under the whole town
Fine river-bottom silt that gives way and lets a normal-set post slowly sink and lean
The fix is under the ground: we over-dig each hole, pour a wider bell-bottom footing that the silt can't pull down, and set a gravel base so water drains instead of turning the soil to soup around the post. Depth alone isn't enough in silt — the footing has to flare.