I'm sorry I can't tell you why your mother is doing what she's doing. But I noticed no one had replied to your post. So I thought I would. My personal opinion is that it's never really possible to know what's going on in another person's mind unless they choose to share it openly & honestly. (And how often does that happen?)

To my mind perhaps the real question here is why are you still with someone who has choked you, & in other ways been physically abusive, as well as continuing to live with a mother who apparently places more value on your abusive boyfriend than she does on you? So my thinking here would be that, rather than spending mental energy trying to figure out what's going on with your mother, perhaps what you need to be thinking about is how to extricate yourself from the potentially dangerous situation you're in.

Having written that though here's a link to a blog, here on PC, that perhaps may be of some help in figuring out what's going on :

Knotted: The Mother-Daughter Relationship

My best wishes to you...

P.S. I also wondered if you are aware of the National Domestic Violence Hotline. Here's a link to their website & their phone number:

National Domestic Violence Hotline | Get Help Today | 1-800-799-7233

1-800-799-7233

Please take care...