Maybe she's waiting for him to see a message she wrote in the food, like she's pregnant again.
While rules of drama tend to lean against there JUST being a scene about eating dinner, this sort of thing was totally common in my family - get home late, and your dinner is in the microwave. When I was getting older, I learned to cook and would make dinner for both my parents, who would sometimes both be coming home around 8 or 9, and almost immediately going back to bed.
I'm guessing that she didn't want to keep her son hungry any longer than it took for the dinner to be ready. That she went as far as to neatly prepare her hubby's dish and keep it warm (it's steaming from under the lid) shows that she actually cares for him, IMO.