The theory is that a Romney win would be risk-positive and USD-positive, which would suggest a double-whammy topside for USD/JPY. Remember that the market is still long (albeit at reduced levels) so it could be a case of buy-the-rumour-sell-the-fact.

Of course the flip side is that an Obama win should be USD/JPY negative, mildly risk and USD negative, and triggering more stop-loss selling.