What kind of reaction is this?
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.