
I think it's because Hori is more comfortable acting like her true self in front of others. In the beginning she acted like a "popular girl" in school, but the "housewife" type (couldn't really describe it another way) at home which supposedly contradicts each other. But even around her friends she couldn't be her natural crazy self.
So now that time with eeryone has passed and they've gotten to know her better, she doesn't feel as restrained.
Hope this actually made sense.

haha
Thanks for explaining but I'm still a little confuse. I understand that she's being more open about herself. I'm just a little frighten by this side of her but regardless her feelings for him is true.
Its just a drastic change. I don't understand why she can't tell her friends that she stay home and do housework? Its a good enough reason plus its nothing to be ashame of... plus it doesn't seems like they were the type to really make fun of her... and if they leave her for that then they're really not her friends. I personally didn't go out a lot in high school to take care of my 4 younger sibling and my friends understood it pretty well. Possibly she is afraid that her friends would think she's lame and make fun of her but the Hori I see at school doesn't seem to care so much about her popularity (althrough she really is)? It just seems a little contradicting.... I can't fully understand her fears, but I can accept them.
As for being her self... from what I can see... Hori sometimes doesn't realize that she's being scary... for example her menacing laughter when she's watching horror movie. I didn't get a sense of this side at all in the beginning. I want to say that possibly she was hiding it from him.. but he comes over to her house like everyday... Natural habits are not that easy to break esp. if she sees him everyday at school and home. I'm sure some parts will slip.... realistically speaking. I felt like the author develop this new personality for her as comedy relief... after they started going out but a lots of things... just stop making sense (but still believable).
However like I say, the story is still good. I'm just not use to seeing this side of her (haha maybe I'm wrong about all this) but I'm happy just seeing them together.

I think she is more selfish now because she feels comfortable, also because Miyamura spoils her. Before, she had to act properly at school (popular girl) and at home (good daughter and sister), but with Miyamura and the development of her friendship with others she is able to be more like herself: more selfish.
I also think she is more violent and aggressive (and it makes me feel uncomfortable almost all the time), as @hollybell said, it might be because she isn’t restraining herself as she used to.
I think her behavior isn’t that strange in her character, due to her mom’s personality and the way both treat her dad. Hori was mean to Sengoku when they were children and Sengoku’s dad doesn’t like her family, because they seem to be sadists.
Hori’s parents showing more, might have to do with there being more chapters in the night time and Miyamura's sleepovers (I’m not sure if it is like that, but that’s what I assumed; also, I don’t remember what time did the first chapters showed…). And with Hori’s dad being at home, it might be because he is unemployed/is in a recess/isn’t working in another city (I forgot his background…), so that’s why he appears much more (I think probably more than Hori’s mom).

I think it has to do with the whole Japanese culture thing. Putting on a face is really common for everyone to do. I have a school face, a home face, etc. Hori's faces are just blending together now that she knows her friends and Miyamura don't really care. Reputation is a big thing over there, even if you have a reputation of being a delinquent.
And honestly, I'm pretty similar to her. So I kinda understand her situation. If I had a guy like Miyamura around, I'd start to get pretty lax with keeping up appearances because he'll accept me either way and will always have my back.
Man. I want my own Miyamura..... ╥﹏╥

The parents are at home more cause of Miyamura. I think that they stated that Miyamura was everyone's Miyamura.
Wow. Hori is a lot more different than how I thought she would be. I still like her but I thought she would be more selfless since her parents are always busy with work. So is it just me or is Hori getting more violent, aggressive, and a little sadistic? I still love them a lot but I kinda miss how they were at the beginning.
As for Hori's parent, won't they suppose to be busy all the time? I feel like they're here all the time now. lol.
Dealing with the Hori's family, shindou, and the kouhai.... I wonder how much alone time Miyahara really have?