Mustrum Ridcully was made Archchancellor in his absence because no one else wanted the job. Several of the recent encumbents had had involuntarily short tenures (promotion in the Unseen University was due to the 'Retirement' of your superiors).

The staff of the University were just suprised when he actually showed up. However they soon came to appreciate that Ridcully gave them a stability that they had been lacking ('retirement' attempts on Ridcully either bounced or backfired).

He has been running the Unseen University for some time now. This is a process that involves telling the staff very loudly what he wants and then ignoring the fact that they do what they want (and also ensuring that the Bursar has enough dried frog pills).

Ridcully comes originally from Lancre in the Ramtops - home of the witches (Granny Weatherwax, Nanny Ogg, Magrat, and now Agnes/Perdita). He apparently was very close to Esme Weatherwax when they were both younger.