Analysis of@Chandra Xray data shows what triggered the Kepler supernova.

"The new Chandra analysis shows that the Kepler supernova was triggered by an interaction between a white dwarf and a red giant star. The crucial evidence from Chandra was a disk-shaped structure near the center of the remnant. The researchers interpret this X-ray emission to be caused by the collision between supernova debris and disk-shaped material that the giant star expelled before the explosion."

Video simulation of Kepler supernova explosion:
http://chandra.harvard.edu/photo/2013/kepler/animations.html

More info, images: http://chandra.harvard.edu/photo/2013/kepler/