I had that happen too and I couldn't understand why. I've also had my Sims had other furious reactions that I couldn't account for. They were so strange.
I once had a Sim furious at his wife for something the Cow mascot did with another Sim in the hot tub. She was in bed with him sleeping when I heard that breaking heart noise that happens when a Sim cheats in front of their significant other. When they both woke up I checked his memories and saw that he was furoius with his wife for cheating with the Cow. She never even met that darn cow and it wasn't her that cheated. I saw the incident with a Cow and another Sim but somehow or another my Sim thought it was his wife. It was so messed up.
"Have you ever had a dream that was SO REAL-"