Are you talking about your own URLs? Otherwise, how could you possibly know that they're being rewritten? Even the googlebot can't tell.
Or did you mean redirect? That would mean that an unusual lot of outdated URLs are in the index. Or that something went severely wrong with the latest update and they've been forced to use some older index. (Hm, interesting speculation. Is each consecutive g### index a discrete dataset that can be stored somewhere? You'd think there wouldn't be enough space on the planet to hold them all :))