"Into The Wild" What happened to cause Chris’s relations with his parents deteriorate mid-way through college?

Chris' relationship with his parents deteriorates over time, as he is angry.... he feels betrayed. In the end, we learn that Chris' anger began with a discovery he made on his road trip after graduation, when he had gone to the neighborhood where he had spent his earliest years. Here, he gets enough information to piece together that his father had continued his relationship with his first wife in secret long after falling in love with Billie and fathering Chris, even fathering another son with her.