Aiming, According to the press release, it was the DSMB that recommended adding more patients. There were no safety issues. As Myostrain said, more subjects will make for a better statistical analysis. Once they confirmed there were no safety issues in the interim analysis, they could safely allow more subjects into the trial.
Hopefully Neuro will chime in soon with his take. He's probably at the BIO 2008 conference right now.