Why Arthur's Mother...? (Spoiler)

A guess, unsupported by anything explicit in the movie, but absolutely consistent with the characters, and the way the actors play the scenes:

She's in the hospital with a terminal illness, probably cancer. She kills herself when she gets home, rather than suffer through whatever few months they've given her.

This explains why she chooses to finally tell Arthur he deportation story, and why Arthur's father is so calm when he delivers the news. And eventually Arthur would learn this, or figure it out, and realize that Baya's bringing up the past had nothing to do with it.

That neither his mother nor father would tell him what was actually going on with her hospitalization is completely in character, and they both play those scenes as if there was indeed a big secret they were keeping.

Prepare your minds for a new scale of physical, scientific values, gentlemen.