Sunlight #1. The side of the stack that gets the most or all the sunlight will lean out. That side shrinks first, the back doesn’t. Most of my stacks run east to west. That gives me a south and north face. They always try to head south To correct I only stack two rows on a pallet, narrow side usually about 42”. I run the north side plumb and the south side gradually leans into the north side for a top that’s 36-31” wide. Fill between the bottom with uglies until just two splits wide. Stacks haven’t shifted at all since I started doing that. I also like to stuff some scrap lumber pieces under both outside edges of the pallets. Nothing in the middle. With time and weight the pallets sag in the center keeping both stacks falling into each other as opposed to away from each other. Frost can play a role too. One side thaws faster. There’s that south side again. The north stays frozen. That’s shifting some serious weight. I’ve seen frost lift soil 2” here in Ct.
Few years ago in a cold snap; water line (under road) froze and broke at a depth of over 8 feet Town subbed it out because no back hoes they had had enough reach… My only comment was insulate it with polystyrene
Chipmunks are destabilizing the earth under the stacks causing them to lean and gravity finishes the job.
I'm on a 3 year plan with our wood. I feel I stack pretty good. Cut all 14" and toss any splits that aren't stack worthy into the ugly totes. Some stacks always start to lean around the 3 year. So I'd say the drying has something to the leaning. They towards the sunny side.
My stacks lean from drying out on one side faster than the other. One of these years ill stop going too tall with them.
I have been trying to make better stacks. I typically do not lose them sideways, but have some ends collapse a bit. I'm fairly stingy on what is allowed to go to 'the good stacks' so not too many odd shaped sticks in my piles. It does make for quite a lot of a-hole pieces. Recently I made a huge jenga 'stack' out of them. It's standing, for now. Woven like a basket. lol
They lean south because the earth spins and flings everything toward the equator, everybody knows that! same reason rivers run south....