Why is pickling considered fermentation?

When you are going to pickle something like cucumber (or cabbage or courgette)which has a high water content it is normal to leave it in brine for a few days and it does indeed ferment, but the bacteria (lactobacillus) are fermenting the sugars present in the vegetable to lactic acid and this helps the preservaton process because the sugars are not then available for spoilage bacteria.