What kind of reaction is this? I have made elderflower 'champagne' a number of times, but I don't really understand it. I ferment other drinks and foods, and generally understand the process quite well, but I'm not sure about this. 
I'm wondering if it is-


some sort of yeast off the elderflowers
A kind of reaction similar to vinegar and bicarbonate of soda (I assume not)
Some other type of symbiotic fermentation/wild yeast or something I haven't thought of. 


The recipe includes elderflowers in a gallon of water with 1 lemon, 1 1/2 pounds of sugar, and a tablespoon of white wine vinegar soaked together for 24-48 hours and then bottled for 2 days for carbonation.
It is certainly a fermentation. A simple chemical neutralization wouldn't need that much time, and wouldn't produce lasting fizz. 

You can be certain that there are wild yeasts everywhere: in the air in your kitchen, on your elderflower, probably on your utensils, etc. It is impossible to say which ones prevailed in the fermentation, the ones you brought in with the elderflower or the ones which came into it from some other sources.