Why does Ronald Frobisher hold such a grudge against Rudyard Parkinson?

Rudyard Parkinson tutored Ronald Frobisher one semester at Oxford, and so when Frobisher's parents came to campus for his graduation and they ran into him, he introduced them. His parents were common people, his father a skilled worker in a foundry, but the more Parkinson chatted away the more nervous Frobisher's parents became, and his father coughed to hide it. Having recently had his teeth removed, he actually coughed his top set of dentures right out, caught it, and shoved it into his pocket, which horrified Parkinson. Later, in reviewing one of Frobisher's novels in which a character was based on his father, he alluded to the scene, which was not in any way mentioned in the book, and for publicly humiliating his father Frobisher has never forgiven him.