I don't know the cause of this large drop and to be honest when I saw it I was pretty upset for a moment cause I never sold none of my UPX that I am mining since december for the reason that I treat this wonderful project not as a cash cow for the moment but more as an investment for the future.
That being said when I saw the actual difficulty I just added a couple of miners from other projects to UPX and I couldn't be happier seeing the results. Never thought this kind of difficulty was possible anymore on such a great ckin.