Why Better US economy means lower SENSEX !!
As the US economy improves, Federal Reserve (the US central Bank like RBI is for India), will stop injecting Billions of dollars it is printing and injecting every month.
Thus when this printing of dollars stop and interest rates are raised by the Federal reserve it will lead to reduction in supply of dollars as well as an increase in cost of dollars (i.e. if you take dollar loan in US it will be costlier than it is today)
Now, as the data coming from US economy is beating expectations, markets fear that the Federal Reserve may soon raise the rate of interest or stop printing fresh dollars, hence people are buying dollars which is leading to a rise in price of dollar and thus fall in other currencies simultaneously.
Now, as a Foreign investor if you invest in Indian equity markets, you first have to invest in Rupee i.e. you buy Rupee before buying shares in India and thus when you believe that the Rupee is about to get weak then you will sell shares and convert your rupee into dollars, thus a fall in equity markets all over the world today for the same reason.