I haven't heard of that particular trigger. I wonder if it has to do with the trigeminal nerve (which plays a big role in the mouth) -- yours may be particularly sensitive, and the vibration of the toothbrush sets it off.
Even though I'd never heard of this, it wouldn't surprise me. Personally, I have found any sort of flickering, visual, auditory, sensual will send me into a partial or aura. The vibrations in an electric toothbrush are quite intense and quite close to the brain. I've never been able to use an electric toothbrush. They make my nose itch then I get queasy.
I think it can be possible because any type of visual, auditory, etc can at times trigger simple partial in me. Yesterday for example I was in the grocery store, already having a hard time with the lighting, but this kid was standing in the aisle with a long stiff foam-like tube and whirling it around really fast over his head, and it made the most high pitch noise as he was whirling it up in the air.. It really bothered me.. not a simple but I think if I would have kept watching and listening yes for sure because I already felt an aura so I guess I did have simple partial.