1. A unionized factory worker performs the same sequence of operations over and over to achieve a standardized type of output. An agile software developer is generally combining operations in new ways: if it's the same thing over and over again, your developer is not very good at figuring out how to reuse code.
2. The nature of the employer relationship is different, at least in today's economy. A typical software engineer is reasonably hard to replace and can apply skills at a variety of alternative employers. The best case for a labor union is when the opposite is true.
3. I think the causal relationship is reversed. The reason that agile is adopted is not to protect programmers from off-shoring or long hours (typical union pitches). It is adopted because the firm is willing to believe that the co-located team with decent hours is a better deal. The union/management relationship may be starkly adversarial; the agile-team/management relationship far less so.
So if there's a UCD fad going around, I don't see it as capable of disrupting the "agile" ecosystem's employees' lifestyle the same way that some new process or external shock could disrupt a unionized manufacturing ecosystem's employees -- not unless it really fundamentally changes how a coherent piece of software ends up finding itself structured, and a couple of wireframes and the like aren't going to do that.