Drive any older Houston neighborhood and you will see it: fence lines that wave instead of running straight, posts tipped a few degrees off plumb, gates that scrape the concrete on the way open. It almost never comes from bad pickets. It comes from what is happening a couple of feet underground.
Houston sits on expansive clay, the stuff people call gumbo. It absorbs water and swells during our wet months, then gives that water up and shrinks during a dry spell. That movement is powerful enough to shift a house foundation, so a fence post is no contest at all. When the clay swells it grips the post and lifts, and when it shrinks it leaves a gap and the post settles somewhere slightly different from where it started. Do that a few dozen times and the post is no longer plumb.
The fix is depth. A post that only reaches into the zone where the soil is actively moving gets moved with it. A post set deeper, past most of that seasonal activity, is anchored in ground that stays comparatively stable regardless of the season, so the fence line stays where it was set.
Concrete matters as much as depth, and it matters in a specific way. The concrete should form a footing around the post that is wider at the bottom than at the top, so swelling soil cannot get under a shoulder and jack the whole assembly upward. A crowned top on the concrete sheds water away from the post instead of holding a puddle against it, which is the other thing that kills wood posts early.
Curing time is the step that gets skipped when a crew is racing to finish in one day. Concrete that has not set is not holding anything, and hanging heavy panels or a gate on a green post is a good way to pull it out of plumb before the fence is even finished. Setting posts, letting them cure, and coming back to build is slower by a day and it is the difference between a fence line that stays true and one that does not.
Gate posts get all of this and then more, because a gate is a lever that works on its hinge post every single time somebody opens it. Bigger post, deeper footing, more concrete. A sagging gate is almost always a hinge post that was set like a line post.