Water, almost every time. Saturated soil is heavier and pushes harder, expansive clay swells against the wall as it wets, and a wall built without gravel backfill and a working drain carries pressure it was never sized for. Undersized structure and surcharge loads finish the job.
More detail
A leaning wall is a physics report. The pressure on a retaining wall comes from the soil behind it, and that pressure is not constant: it rises sharply when the soil saturates, both because wet soil is heavier and because water that cannot drain becomes hydrostatic pressure pushing directly on the wall. North Texas adds its own multiplier, because the Blackland clays on the east side of the corridor swell as they take on water, loading the wall again with each wet season and relaxing each drought, a cycle that ratchets an under-drained wall outward year over year. The failure chain is usually built in on day one: native clay packed against the wall instead of free-draining gravel, no filter fabric, no perforated drain (or one that never daylighted), and surface grading that sends roof and slope water directly into the backfill. Add the structural shortcuts (a footing too small for the height, no geogrid on a tall wall, a wall built casually under a surcharge like a driveway or slope above) and the lean is only a matter of seasons. Frost heave, the classic northern culprit, is a minor player here; our freeze depth is shallow. The diagnosis matters because it writes the fix: a rebuilt wall gets the drainage and the sizing the first one skipped, which is why rebuilds here itemize the invisible system, not just the visible face.