What does Charles say is his reason for visiting Mama?
Charles simply says to Chig that they might as well drop in on his mother as well as his brother. He says it so casually, it all seems so sudden. And the fact that the proposed visit wasn't in their original itinerary merely adds to the surprise. Given the incredibly fraught relationship between Charles and his mother, it's not immediately apparent why Charles would want to stop by. Certainly, when Charles shows up at the house and gives his mother a kiss, it is perfectly obvious to Chig that there is something not quite right about this mother-son relationship. Chig can see all kinds of emotion in his father's eyes—fear, uncertainty, sadness, even perhaps hatred.
Perhaps Charles wants to visit his mother because he has so much he wants to get off his chest. During dinner, years of pent-up anger, rage, and resentment come rising to the surface. We sense that he has waited a long time for this moment, when he can now say all the things to his mother that he has wanted to say for so many years.