What happened to Curly that later gave Buck nightmares?

In chapter 2, Curly approached a husky in a friendly way, but the husky took this as an insult. Many huskies came and circled around Curly, and when she fell, they attacked her and got her killed. The sight of watching Curly being killed brought Buck many nightmares to come. Hope this helped! :3

What was causing buck to have nightmares