Why did wooden furniture parts warp after staining but before assembly?
You're right assembly &amp; locking into position would've likely prevented most of the warping, because Ed Beal's right. Though, longer spans would've possibly still bowed. The wood today is all new growth &amp; very unstable compared to old growth stuff. Even tightly strapping &amp; then heavy weighting wood together until it's dried or acclimated in a week or 2 is still far from predictable.

But after that, sanding or planing it back to reasonably flat is a better practice. Which, would be your solution at this point, for just the areas that matter most. Using Furniture Grade plywood avoids much more warping to yield better results, if you don't want to get a PhD in wood storage, curing, steaming &amp; handling.