Why did Lainie's mother get mad at Lainie for starting a fight with Joel when he's the one who pushed her?

Lainie's mother was absolutely awful. Even though Joel was a terrible boyfriend Lainie's mother seemed to have some sort of bizarre cush on him & wanted to live vicariously through her daughter. She even straight out tells Joel that she wants him to become "part of the family" while plotting with him to get Lainie "under control" after she starts losing interest in him & becoming distant. She inexplicably pressures her daughter to date Joel even after it's clear she's not interested in him, while being rude and hateful towards the guy she actually likes for getting in the way. Even after it's revealed that Joel is a controlling, physically abusive alcoholic she blames her daughter for "provoking" him. To top things off, when Lainie won't "listen to reason" in her mother's eyes, she gets her sleazy mustache boyfriend to straight up drug her with drugged wine, which quickly makes her pass out.

So yeah all around, terrible, unsupportive mother who has no qualms about even jeopardizing her daughter's health.