Is my honey fermenting? I bought a jar of honey a few months ago, and left it at the back of the cupboard. When it resurfaced a few days ago, I saw that the jar contained a lot of bubbles, to the point where the lid was bulging. I assumed that it had somehow fermented, particularly since it came from a local grower, where taste can be expected to take precedence over sterilization; this answer seems to support my theory.

On the other hand: the jar was still (to inspection) airtight, and when I opened the jar there was no gas release; the honey gently oozed over the top, and remained inert. The bubbles have remained in the same place over the last week, and the honey tastes exactly as normal.

Clearly some sort of chemical reaction has taken place; is it classed as fermentation, and is it harmful?

(It is almost impossible to take a picture of this; the bubbles are equally distributed throughout, giving the honey much the apparance of the interior of an Aero bar. If you were eating it with your eyes closed,  I don't believe anybody would notice any difference).
Yes, your honey started to ferment. 
I assume the fermenting process is still fresh as you mention the honey taste didn't changed (later it change taste to beer-like). With time yeast can produce alcohol or vinegar from carbs in honey. 

You can still use this honey for baking or making mead. Unfortunately the only way to stop the process it to heat honey to 80C to kill yeast.  

The fermentation in honey can be due to gathering honey too soon so there's to high amount of water in it or not properly creaming it.