NEW YORK ( TheStreet) -- Short interest on financial stocks rose 9% in the second half of May, the largest increase since early March 2009, when financial short interest rose 24.6%, according to research published Wednesday by Keefe, Bruyette & Woods. The surge, encompassing the period from May 15 through May 31, was driven by a big rise in short interest on "universal" U.S. banks Bank of America ( BAC), Citigroup ( C), Goldman Sachs ( GS), Morgan Stanley ( MS), and JPMorgan Chase ( JPM) which together saw a 61% upswing in short interest. Particularly notable was a 75% rise in Bank of America short interest and a 61% surge in Citigroup short interest, KBW's analysts observed. During that same time frame, Bank of America saw its share price rise 0.7% while Citigroup shares fell 4.6%. The S&P 500, meanwhile, fell 1.5% during that same stretch. Short sellers borrow shares in the hope the stock will fall. They can then buy it back at a lower price and pocket the difference. The New York Stock Exchange releases short interest data twice a month with a roughly two week lag time. -- Written by Dan Freed in New York. Follow this writer on Twitter.