From New Scientist Tech:
A female soldier tried to free the shell, but another shell was accidentally fired, causing some rounds in the gun's two near-full ammunition magazines to explode. The gun began firing again and swung in a circle, leaving nine soldiers dead and eleven wounded.
Blogs and other online news sources have suggested the incident may be due to software problems, highlighting the danger of automated weapon systems. But Jim O'Halloran of defence publication Jane's Land-Based Air Defence says the incident is more likely the result of a simple mechanical failure.