CTA positioning is too short, and they are forced to cover in pretty much any scenario (barring some major negative market dislocation) according to the info provided. 

Keep in mind, corporate buybacks in black out period - resumes April 26, and there is massive amount of catch-up to take place, relative to annoucements. Soo, there is another bullish driver.

I commented on this last night, bearishness was too high and not enough big players were positioned long, too many hedges as well, which is why I said I was not looking for the market to breakdown in March when it had pulled back in February, and even last week too much bearishness