I thought the AAIB only investigated serious incidents and accidents. I don't think an accidental slide inflation is that serious.
This type of incident, causing injury would be classified as and accident, therefore the AAIB are duty bound to investigate. As with most things in aviation there is a very clear definition of what constituted an incident, a serious incident and an accident.
If you do your ATPL air law you will find that this area is a favorite of the examiners, with a few questions on the subject.
On a different note I'd be interested to know how this happened. Doors opened from outside (if it was) should automatically disarm, and if opened from the inside whilst armed, then the slide should drop out onto the steps, but unless the steps are a good few feet below the sill then the slide shouldn't inflate as it won't drop far enough to pull the lanyard.