When I take my floppy-eared dog to be groomed, he sometimes comes back shaking his head like his ear is bothering him. He does occasionally get ear infections. Is there any connection between his grooming and ear infections?
Most outer ear canal infections are caused by overgrowth of bacteria and yeast. Water in the ear can create the moist environment for this to happen. Another possible cause is debris that gets stuck in earwax that causes irritation, inflammation, and infection.
So when your dog gets a bath, he may be getting water in his ear that starts an infection again. Also, while drying, that warm floppy wet ear covering the ear opening may create enough moisture to cause bacteria overgrowth. Or, when he is trimmed, hair could be falling down into the ear canal, again sparking onset of another infection. Keep a supply of medication from your vet on hand when these early symptoms arise.
A way to prevent problems in the first place is to keep the water and debris out of his ear canal. Since he is a floppy-eared dog, I would suggest putting cotton balls in his ears during bathing, drying, and when they clip the hair around his head and ears. This way you can remove two possible situations that might trigger another ear infection.