Curly died from being attacked by a pack of dogs. Curly’s face was ripped from eye to jaw and then she was surrounded and eventually killed. 

Text Evidence:  

In the text it said, “There was no warning, only a leap in like a flash, a metallic clip of teeth, a leap out equally swift, and Curly's face was ripped open from eye to jaw." 

 This was what the onlooking huskies had waited for. They closed in upon her, snarling and yelping, and she was buried, screaming with agony, beneath the bristling mass of bodies.”