Why does Buck decide to leave the Baxters so abruptly in The Yearling?
I am assuming that you are referring to the incident in Chapter 24, when Buck comes over the the Baxters' place to invite Penny to join them in hunting wolves the next morning at dawn. The wolves had been heard howling from neaby the Forresters' Island, and Buck knows that "with luck, all the remaining wolves might be killed (and) game also could easily be taken." Appreciative, Penny accepts the invitation, and asks Buck to spend the night, but Buck says that he must leave. He explains to Penny that if he does not show up back at home by bed-time, his brothers will figure that there will be no hunt, and will not be ready early the next morning.
The other incident to which you might be referring occurs in Chapter 25. When the hunt was nearly over, the Baxters and Forresters had come upon the tracks of a big buck deer, but had agreed not to go after it that day. While on his way home, the buck had run across Penny's track, and Penny, taking advantage of the opportunity, had killed it. When Lem sees the hide stretched out at the Baxters' place, he gets angry because he thinks that Penny has betrayed the Forresters, going after the buck on his own so he could have it for himself. Penny tells Lem the truth, that he came upon the buck by "pure happen-so," and Buck sticks up for Penny as an honest man. The Forresters, and Buck in particular, seem distant after that, and when Buck finally comes by again to tell Penny the wolves have been eradicated, Penny says,
"I hope you ain't mixed up in your mind about that buck Lem got so ornery about."
Buck evasively says,
"That's all right. What's one deer?"
and leaves quickly, indicating that perhaps he does believe that Penny had been trying to take advantage of the Forresters by going after the buck on his own, and is understandably angry and hurt.