A.E.
I think part of it is the age. They are preadolesent. Some 9 year olds are starting to develop, their bodies are changing as well as hormones. At least that seems to be the case with my 9 year old and her cousin. Same deal both sweet girls, and do well in other situations but at home can snap if you look at them sideways. Hopefully it is just a stage. I'm trying to give my daughter extra praise and love but still try to follow through with discipline as needed. If you haven't talked to her about why she is starting to have mood swings etc. I know you didn't mention body changes but I just introduced my daughter to the American Girl book The Care and Keeping of you. I also noticed they have several books on emotions etc that I will probably be looking into.
Good luck