Why is my mother acting like the child in the family??

I'm not sure where to start but my mother has been absolutely unbearable lately. When I moved out a few years ago, I was unable to take my large dog with me to my apartment, so she agreed to keep him at her house where there is a yard for him. I have remained financially responsible for him, and agree that this is only fair. He is sick and may have to be put down soon. Over the weekend, I stayed at her house to keep an eye on him just in case things got worse. He isn’t getting worse, but not getting better either. My mother seems to think this is her opportune time to go back to her old ways of manipulating me, which she did ALL THE TIME when I use to live with her years ago. My best friend was over during the weekend and noticed her behavior which included intentionally trying to embarrass me in front of her, putting me down, insulting me, and then she also had a full-blown meltdown because she couldn’t get her lawnmower to work. I had intended on having a nice afternoon with my friend and mother since they hadn’t seen each other in quite some time. Instead, it was a disaster, and the last time I invite her over again to my mother’s house. We aren’t children anymore, and both have full-time jobs, money, our own cars and apartments, and our own lives. My mother seems to think its okay to treat us like we are back in high school.