The brake-tapper is a particularly aggressive and/or fast driver who comes barreling up to the car ahead of him/her and tail gates at such a close proximity that he/she ends up tapping the brakes to avoid rear-ending the poor schlub in front. This usually goes on until one party turns.
Followers behind the brake-tapper should take note that just because he/she is hitting his/her brakes, those at a safe distance don't necessarily even need to brake themselves. It is especially annoying and careful attention is imperative.
You: What the damn, why are we braking?
Me: We aren't. That guy in front of us is just a basement-dwelling neckbearded brake-tapper.