If it's just happening intermittently and not staying in a retracted position, it could be she has a piece of food stuck between her teeth that she's trying to work out of there, or possibly some other dental issue. Though to me, this doesn't look as likely based on your video, with her not chewing and moving her mouth around while the cheek retracts. But still possible.
If it's consistently staying in a retracted position on the one side (drooping on the other) or continues to happen a lot, this can happen with a middle ear infection affecting the facial nerve. It would be the most likely cause I know of, when a rabbit presents with one side of the face consistently being in a retracted or drooping position. There are other possibilities, but ear infection seems to be the most common one.
If this is what's going on, it means that the antibiotic ear drops given months ago, just knocked down the previous infection, but wasn't a long enough treatment or aggressive enough (no systemic antibiotic prescribed) to clear the infection up completely, and now it's re-emerged and progressed to the middle ear and is affecting the facial nerve.
My opinion is that antibiotic ear drops for a couple of weeks, is entirely insufficient to treat an ear infection with a rabbit. Ear infections can be extremely difficult to clear up in rabbits, especially if it's spread to the middle/inner ear. Antibiotic treatment should almost always include a systemic antibiotic (rabbit safe, oral or injection), usually for a minimum of 4-6 weeks, and sometimes much longer, depending on the severity and the antibiotic being used. In my opinion, a vet that only prescribes ear drops and no systemic antibiotic, is likely not very experienced with rabbits. An anti inflammatory pain reliever (meloxicam) should also usually be prescribed, as it helps reduce the inflammation and pain caused by the infection, and may help prevent permanent damage occurring.
www.medirabbit.com/EN/Neurology/Otit/otitis.htm
This is just a possibility, it could be related the the mites, or ivermectin if overdosed, though I haven't come across any mentions of that type of behavior or side effect happening in other rabbits, with either of those possibilities. And it's also possible it could be something else entirely.
If this keeps happening, gets worse, or you think it could be the ear infection that's come back, my recommendation would be to find a more experienced rabbit vet to go to if at all possible, even if you have to drive a ways to get there. A really good vet can make all the difference in getting a proper diagnosis and treatment.
Do as much research as you can on any vet you're interested in. I particularly pay attention to personal accounts from other rabbit owners, as well as vets that rabbit rescues go to.
https://wabbitwiki.com/wiki/Choosing_a_rabbit_veterinarian
