There are some people, namely mentally unstable individuals or those under the influence, that can be unpredictable in what they will do. Alcohol heightens emotions, and they may randomly get angry and seek physical conflict with you. No reasoning will stop them at this point. Many things in life are also unpredictable.
However, among the top of unpredictable things are altercations on the streets, and even combat sports in general. It only takes one punch to take your lights out, and it can easily happen when the circumstances are just right. When it does take your lights out, not only will you get a massive concussion, but you will also suffer a potentially fatal blow if you fall on the back of your head on a hard surface.