All dogs are prone to ear infections, especially dogs with "floppy" ears and lots of hair =)
Although yeast and bacteria (malassezia and cocci usually) naturally live in the ear in small amounts, moisture can accelerate growth of yeast and bacteria within the ear canal and the overgrowth will cause the infection. Other causes for ear debris include ear mites, and rare systemic problems. However, most ear infections are the result of environment (swimming dogs, dogs that go to the groomers a lot, often bathed) and become chronic because they either are not properly cleaned/treated at home and the environmental causes are not taken care of.
Make sure you recheck with your doctor and that the ear is properly scoped to make sure the ear canal is free of debris. Even the great doctors I work with are sometimes guilty of not scoping the ears if the pinna looks great during the recheck, they just came back from the Western Vet Conference with renewed vigor on scoping ears after a lecture on chronic ear infections.
Good luck! :wave: