To abbreviate the message from SpaceX CEO and founder Elon Musk reports:
"The problem arose due to the longer thrust decay transient of our new Merlin 1C regeneratively cooled engine, as compared to the prior flight that used our old Merlin 1A ablatively cooled engine. Unlike the ablative engine, the regen engine had unburned fuel in the cooling channels and manifold that combined with a small amount of residual oxygen to produce a small thrust that was just enough to overcome the stage separation pusher impulse... As it turned out, a very small increase in the time between commanding main engine shutdown and stage separation would have been enough to save the mission...It looks like we may have flight four on the launch pad as soon as next month."
COMMENT: As others have pointed out, SpaceX needed only days to identify a cause and post the cause and intended fix for the whole world to read. NASA should learn something here. Good luck next time to Elon and company.