There can be a number of things and sometimes several at once. Ear mites can live in the ear and not only cause irritation, they can also predispose a dog to secondary yeast or bacterial infections. Ticks from time to time may crawl into the ear and cause irritation and depending on the tick, can be very dangerous. Yeast and bacteria, often found growing as a natural and benign flora on a dog’s skin can, under certain conditions, flare up suddenly, causing ear infections. Dogs that suffer from yeast and bacterial ear infections may have an allergy problem or a genetic predisposition to having yeast overgrowth issues. Foreign objects like dirt, grass seeds, or anything else that will fit, can become lodged in the ear canal and cause irritation and infection. Even swimming can be a cause. when water gets in the ears and can't get out, it provides an ideal environment for microbes to grow.