Originally Posted by
spock
..The wheel seemed to have been built fine when I walked out. Tension seemed equal and strong enough and the true was good. I think the spokes simply loosened.
Are you familiar with the concept of spoke wind-up? It's when turning the nipple, it stays put in relation to the spoke, so you end up twisting the spoke instead. This also increase tension in the spoke, so the wheel might still
look fine and dandy.
But subject it to a few road bumps and the spokes will unwind and lose tension. If you hear a pinging noise when first riding a wheel that's been worked on, this is probably it.
Although more prominent on butted spokes it can happen to any spoke.