People throw knives on the street . . . doesn't happen often, and almost never with throwing knives, but it happens. People are idiots, add chemicals or alcohol, piss them off, and they might just throw a lockblade or sheath knife in your direction. Often, they actually hit the person they're aiming at. Fortunately, it is seldom with the point. I recall reading a report of some dirtbag in Binghamton who died after another dirtbag threw a knife at him from over 15 feet away. I'm fairly certain that the thrower had no skill -- he just happened to sink the point into a vital area by dumb luck.
Respond to a thrown knife the same way you'd respond to a thrown bottle, brick, or billiard ball. Get out of the way, or grab a trash can lid or briefcase to use as a shield.