I remember this hapening but can't remember what actually happened. All I can remember was that the Airfield Fire Service stood the AGI at either 22 or 42 minutes past the hour (not quite sure why I remember that - perhaps I don't as I may be wrong). I don't recall any passenger evacuation so presume the aircraft was empty due to a long turnaround. Your picture does show a set of engineers steps at the rear of the aircraft, so perhaps it was something to do with the APU (purely speculation).
All incidents such as this are attended by all, that's why you get AFS, rangers, police, airport paramedic etc.
I know that this doesn't answer your question, but does fill in a little.