There are a lot of nuggets of learning buried in any given course, many of which would be interesting and useful or even really important to people who will never take that course.
And there are, in the course of one's organized education and work, questions we have right now, today, this hour, for which "go take a course on X" is NOT a realistic answer.
I know a music professor who does an incredible piece on improvisation that would be pure gold for my public policy students. When my rapid prototyping students ask why some 3d printer filament melts at a higher temperature, I could say, "Well, that's polymer chemistry. You can take chem 1 and then chem 2 and then organic 1 and 2 and at the end of organic 2 you get to polymers…." But nobody's going to do that. Alternatively, I can call up my colleague in the chemistry department and arrange for a one hour basic introduction to plastics and polymers for tomorrow afternoon.
The popUp curriculum consists of short, independent learning sessions that are scheduled opportunistically. Most commonly they'll be 2 to 3 hour workshops, but sometimes they'll be a series of meetings over a few days or weeks. They can happen because a workshop leader happens to be in town or because an instructor has identified a "right now" skill need. Or we manage to identify segments of existing courses that would be awesome to open to the whole college or community and so we create the infrastructure to make that easy. Or they may occur when a few instructors realize their courses have an overlap that can be "popped out" and shared. Or faculty have identified ancillary skills that make sense at a particular point in a cohort's developmental trajectory but that s/he hasn't been able to fit into an existing course. Or pedagogical auxiliaries such as career services can offer a training that gets extra bang when it can be "co-branded" with the popUp series. Or a field trip opportunity that doesn't fall under just one course presents itself. Or we want to take advantage of non-traditional teaching resources - such as the facilities person who is a master wood worker - that would not fit into the conventions of course-based learning.
Examples from our catalog