@lkptyl It's probably from not fulfilling wants. I'm noticing this too, and it seems to be the sims throwing a fit because you took too long to fulfill wants. This new wants system seems to be poorly executed because there seems to be so many things wrong with it, and negative moods/moodlets is just one issue I've noticed. I'd suggest turning wants off, but even the toggle option doesn't work properly. 🤨
You can try turning off the wants to see if it makes your sims less moody, though be aware that doing certain things will just glitch the option and force them back on even though the setting is still off.