What's taught at universities depends very much on staff interests and expertise. If someone has a specialism within another subject, then they may teach that as part of the subject and possibly be able to offer it at Certificate level or as part of joint/combined honours degree. For instance, one of the history staff at my university specialised in sports history, so we got to do a number of modules in that subject and, as I left, a postgraduate certificate was about to come online.
Usually what happens is that the person (usually a Doctor or Professor) who wants to introduce the subject to be taught sets out a course proposal showing exactly what would be covered, how it would meet the necessary academic rigour and how it would fit into the wider curriculum of that particular university, as well as implications such as equipment and costs. This would be put before a panel of university 'bosses' who would decide whether the course could take place.
If the course is successful, further courses can be developed until there is a strong case for a full qualification in that subject - all decided and moderated by various boards and panels. I'm sure someone who's worked in the HE sector will come along shortly and explain exactly which boards and panels.