What causes ear infections?

When I raised my Sammies... the only issue I remember having were their allergies to fleas and hot spots. So, I've never really dealt with ear infections.

Now, it seems Lola has another ear infection... crusty on the outside, red and a bit raw in the inside. The vet prescribed some steriods to help with healing and she thought perhaps they might be an allergy. What kind... ? she doesn't know.

So, I'm wondering.... what other dog owners have discovered about ear infections. She's on blue Buffalo and she loves it without any stomach discomfort (she never really had any issues with food except for Nutro that turned her skin very pink and itchy)