Why did my dog's ears start to flop?

This answer is part of Pet's Spring Cleaning Campaign. This question is old, but this answer will still help people with the same problem.

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It depends on your dog. For example, one (or both) ears drooping is very common in strays as described by hospitalveterinariglories.com:

There are different conditions that can cause a dog to have one ear up and the other down. However, they are usually accompanied by other symptoms. These other symptoms will generally be more obvious and should lead you to your vet’s office to rule out serious ailments.

When the dog is not treated in time, as can be the case with abandoned dogs, the ear may never lift. Unfortunately, this is often the case with stray dogs. They may have a flabby or misshapen ear due to malnutrition, fighting, illness, fungal ear infections, parasites, or other causes.

Some of the more common causes include:

• Bite wounds: when dogs fight or play, it is not uncommon for their ears to be injured, as they are vulnerable and accessible areas. Animal bites are often complicated by ear infections. Except for minor injuries, they need veterinary attention and even surgical intervention as a means to avoid deformations.

• Otitis media: Otitis is a general term for inflammation or infection of the ear. This can occur only in the outer part of the ear (external otitis), but it can also occur in the inner ear (internal otitis) or even middle (otitis media). For the latter, it is common for the dog to regularly keep its head to one side and even shake it in frustration. It is caused by an infection (usually bacterial) that enters the ear and can affect other parts of the dog’s face if it is acute. Some dogs are predisposed to otitis and, in these cases, they may need a surgical intervention to solve the problem permanently.

This is common in strays and adopted dogs because they are prone to getting infections and fighting.

If your dog was not a stray, he could still have bite wounds, but it is most likely an irritation that was scratched too hard and resulted in an infection, or internal otitis media.

Only a vet would be able to determine the exact cause, so it is important to go to a trained veterinarian immediately (to treat the worst case scenario).