I think the two biggest things that trigger my 3 year olds behavior is being hungry or being bored, mix the two and we are in some real trouble. Which then means if we are out running errands for a couple of hours, and she didn't eat a proper meal beforehand, her snacks won't cut it and she will have a meltdown. She will sometimes be bored running errands as well so then I'm in double trouble. This can happen at home too, if she doesn't eat well I try and do everything I can to get her to eat something to lighten her mood. Within minutes of a good meal she is a totally different kid!
LO2 is pretty chill all around, I think not eating and her sister picking on her are her bad mood triggers. She is usually pretty content with whatever we are doing so doesn't usually get upset if she is bored, she always finds something to do!
What triggers your LO's bad mood?