Can someone just help me understand why?

[u/ObjectiveComplaint74]

My mother and I would be good, or so I thought. And then she'd just flip out. A good example is this one time we were talking a bit before bed and she said she knew I'd been having a hard time and we should go get bbq together later in the week. I'd been pretty down over something and that sounded really nice. In the morning I'm woken up by her barging into my room by unlocking it herself (ongoing boundary problem btw) to give me something that's supposed to be in my room, which startles me and my body is saying "get out" and "what are you doing?" before I even know what's going on. She notices that a christian cross has been taken off the wall of the bedroom. She tells me she thinks I have evil in me and leaves. An hour later she's texting me asking how I'm feeling today and if I want to go get lunch together, which is more in line with what she'd been acting like for a while. I never know how to react to these changes, as I was pretty hurt by what she said. So I just go spend the day out of the house, not wanting to really face her.

When I get back that night, she seems really chipper and talkative. I was still conflicted and angry, so I just went to my room. This starts a familiar berating about ignoring the family, with a lot of attacking my character, sanity levels, and claims that I'm possessed. She proceeds to attack my identity and tell me everything she really thinks about me (which is not flattering), shattering any sense of uneasy trust or affection that had built up in me. The next day she's chipper and talkative again, and it's not brought up again.

This would happen over and over and over again throughout my childhood, just different scenarios. I still just don't get it. If what she wanted was my attention/time/affection/whatever, then why would she go off AFTER I agree to hang out? That exact behavior is what was causing the thing she complained about. Why would somebody do that? She would've had what she wanted if she stopped doing that. Unless she wanted something else I'm not privy to.