My cat attacks my mom but not me

So, February this year, we adopted a 2 year old neutered male. Me (f27) and my mom(63) live alone. Prev owner was an adult daughter of a vet and the vet fostered him until we adopted him.

So far, he's a really nice cat. Little unplayful, but he like getting pets and allows himself to be picked up fine. But for some reason, he seemingly randomly attacks my mom, not in a playful way, but by biting and scratching hard. He has bit and scratched me a few times as well, but this is usually because I held him too long or he got frustrated while getting his nails trimmed.

My mom treats him well, and she's even the one who usually puts the food out for him, but he's always attacking her unprovoked. Often when she goes to take a smoke at the balcony, he'll join her to look down, and they get along well, but sometimes he lays at the doorway on the way in and starts getting really aggressive at my mom when she tries to leave. Strangely enough, he's ok if I come over and pick him up to let her leave.

I'm also kind of worried about how he doesn't seem to give warning signs. He'll do kind of a high pitched meow, but he doesn't growl or move his ears back, so he's really unpredictable.

Other weird behavior he has is that he eats dry food, but he's gotten really picky about eating wet food, and he hates being picked up while facing you or when he's lying on his side. Laying on the ground with his belly at the bottom is fine.

Would really appreciate help on how to get him to stop attacking my mom.