As I like to think of it, Maa Durga, though she was calm, cool and collected, riding into battle laughing at the folly of the demons and kicking demon butts, finally lost her cool, and became so enraged that Maa Kali manifested from Maa Durga's forehead and appeared.Ma Kali is another aspect (for lack of a better term) for Ma Durga.
I once saw a group of young mothers (my own sisters included) reenact the transformation as they descended on a school principal who put children in danger with a bully. The principal was not Mahishasura, but she was kind of an old cow. If a couple of 20-somethng mothers can wreak such havoc I cower imagining what the Mother of the Universe is capable of. 'Twas not a pretty scene.