Why do Gikonyo and Mumbi argue in "Chapter 12"?

Gikonyo is angry about the loss of the farmland he wanted to buy, so he has a bad attitude when he goes home to Mumbi. He snaps at her and shoves her son, which makes Mumbi call Gikonyo a coward. Gikonyo then slaps Mumbi in return and leaves angrily again, never explaining his bad mood to her.