There are no LEDs that directly emit white light. White LEDs are actually blue/purple LEDs that are coated with a phosphor (which looks yellow) to down-convert some of the blue/purple light to the rest of the spectrum. Defective LEDs (likely for street lights that probably use the cheapest LEDs they can find) might have the phosphor layer peel off, making the LED emit all its energy as blue/purple. So there exists a reasonable explanation for purple street lights.