I assume you are talking about what happens towards the end of Chapter 5.
There seem to be a couple of reasons why Buck does this. First, he has lost the will to live. The men have treated him and the team so badly on this trip that the "spark of life" is about to go out.
Second, the book says that he has a sense of "impending doom." I would say that this is what Buck has that the men do not. He senses somehow that the ice is not going to hold them.