Index inclusion should mean an increase in short interests as...

Index inclusion should mean an increase in short interests as part of index arbitrage strategies, so not a surprise and not necessarily an indication of increased pessimism specifically on ING. Also, once a stock is in the index, people wishing to take on a short position on the index as a whole will need to add short positions in the newly included stock - so not an indication of anything specific to ING.

Also, while the academic literature is all over the place, there is some support for the position that increased short interest is followed by periods of outperformance in the underlying stock: http://essay.utwente.nl/67202/1/Schindler_BA_MB.pdf