I think that the expansion in the user experience field followed the growth and spread of computers. This may not be news to other people, and isn’t really a big surprise to me, but until I started the history project, I didn’t think quite so explicitly about the timeline.
UX’s roots go back far, but the human-computer interaction field, probably the start of usability or user experience proper, takes shape around the late 1970s and early 1980s. This field is, for the most part, filled with academics. One of the drivers of this growth is the spread of mainframes and then minicomputers away from academia and out into the business world, where suddenly these complicated machines had to be used by non-scientists. People in computing and psychology noticed suspected that these machines could be designed in a user-centered way, and thus human-computer interaction started to coalesce.
The field stayed mostly academic for much of its first decade. However, going into the late 80s and early 90s, microcomputers (AKA personal computers) came on the scene in a serious way and spread computing even further into the world. There were more and more practitioners working outside of academia to make computers easier to use, and while they took what knowledge they could from the academic CHI conference and from academic papers, there was a movement to start a professional organization that catered to practitioners.
Thus, UPA was born in the 90s, and, in time, other organizations related to user experience.
I still struggle with a good explanation for why a group like the UXPA struggles for membership nowadays. Perhaps it’s the result of our field’s success—with so many practitioners, there are so many opportunities to learn and to network with other practitioners, that there is a necessary fragmentation of professional organizations.