Stocks are now almost back to flat (green line on 15 minute chart, top left). Perhaps this disappointment is responsible. 
excerpt from Zero Hedge:
The April ISM is out, and while it confirms last week's declining  Chicago PMI data and the fact that the Japanese contraction has not even  remotely impacted US businesses yet (and it will), the  recent weakness predicted by various Fed diffusion indices is being  confirmed. The ISM came at 60.4, a decline from 61.2 in March, primarily  a a result of a fall in Production (-5.2), New Orders (-1.6) Supplier  Deliveries (2.9) and Imports (-1.0). All of these metrics will drop far  more once the Japanese contraction is truly appreciated.
