Why does my guitar neck warp?

"Warp" gets used loosely, but there are really two different things people mean by it, and they have different causes and different fixes.

Forward or back bow (relief issues)

This is the most common thing people call "warping," and it's usually not warping at all — it's the truss rod and string tension being out of balance, which is a normal, adjustable condition, not damage. Too much forward bow (relief) causes buzzing in the middle of the neck; back bow causes buzzing up high and can make fretting difficult. A truss rod adjustment fixes this in the vast majority of cases.

Actual twist or lateral warp

Less common, and more serious. This is when the neck develops an actual twist along its length — one side higher than the other when sighted down the neck — and it's not something a truss rod, which only counteracts forward/backward bow, can fix. Actual twist usually comes from wood that wasn't fully stable or properly seasoned before being used, extreme or prolonged humidity extremes, or a guitar stored improperly over a long period.

What causes it

Humidity swings are the biggest driver of both types, because wood moving unevenly is exactly what creates both bow and twist. Cheap, poorly seasoned wood is more susceptible to actual twist specifically, since the wood's internal stresses weren't fully settled before the neck was shaped.

What to actually do

If it's relief-related, a truss rod adjustment (possibly paired with restringing to proper tension) usually solves it, and it's a normal, expected part of setup maintenance, not a sign something's wrong. If it's true twist, that's a more involved repair — sometimes correctable with heat and pressure over time, sometimes requiring a neck replacement in severe cases.

If you're not sure which one you're dealing with, sighting down the neck from the headstock, and checking with a straightedge, is the fastest way to tell before assuming the worst.