Sounds to me like when the posts were set they just tossed the dirt in the holes and if they compacted anything, they had too much dirt in the hole and only compacted the top layers. Pretty much the fix is to redo it properly. Pull the post, make the hole, hold the post plumb, fill in only an inch or two of dirt at a time and hit it with the spudbar to compact it as you go. If you don't compact it as you go the bottom of the post stays loose and the compaction at the surface acts as a fulcrum point shifting the bottoms to the west and the tops to the east. If they are compacted all the way down, it can still lean the fence but the effects are not as quick or drastic. I've seen this a lot in my area with the fences installed by Lowes and Home Depot, they just toss the dirt in the hole around the post and step on it to compact it... I asked one Lowes guy I saw installing one like that a couple summers ago and asked why he wasn't compacting it with a spudbar as he filled the hole, he said "oh first time it rains it will all settle in and be fine.". This past winter laid that fence on the ground...