My 8yo daughter has recently become so mean to my boyfriend

My daughter in the last 6 months has become incredibly aggressive and just downright disrespectful and mean to my boyfriend. He has actively been in her life for the past 1.5years and everything was great! She even called him her bestie and would tell him she loved him. Now, he can't even talk to her without being met with "shut up" or kicking/hitting/punching/pinching or insults. I've tried talking to her, she just says "He's annoying. I hate him" - Edited post to add: She has also broke down crying when I've asked why she is always hitting him and said "I can't answer why I do it, it's not me" and she responded that way 2 times now 😳

Her dad and I share custody, he's very active in her life and our co-parenting relationship is pretty good. I don't think he would be putting negative feelings in her head. I'm also not naive to the fact that kids want their parents together and she just may be growing into realizing that my SO is standing in the way of mom and dad getting back together? I don't know.

My SO has been amazing and patient with her through this change in behaviour. I'm really at a loss of what to say to her to get to the bottom of this or how to deal with her exhausting behaviour. I feel like all i'm ever doing is disciplining her when she's with us and I don't feel like that's a constructive way to handle this. I don't want her to think i'm choosing my SO over her all the time but at the same time, she cant keep acting like this.

Any advice is welcome ):