Husband's anger scares our kids

My husband is not a violent person. He has never even been in a fight. However my MIL springs out a side of him that scares our kids. My husband never yells unless it's to his mom. There are a lot of issues there. His mom had called him to talk about something having to do with his dead brother and every time they talk it turns into screaming. I could hear him screaming from the room. He called her a fucking bitch and told her she treats him like shit all the time and he's not her fucking punching bag.

I don't understand why he continues a relationship with her when all it does is cause him stress. He also has high blood pressure and he has went to the hospital because arguing with his mom raised his blood pressure way too high. I keep asking why does he continue a relationship with her because when he gets so mad it scares the kids hearing their daddy scream like that. He says he doesn't know why he continues the relationship with her. It's only her that he gets this angry with.