Opinions on my boyfriend going off on his mom

I go to my boyfriend's house almost everyday. We are both 18. I get along very well with his mom. Yesterday though he went off on her completely out of nowhere. How it started was I was talking about my grandfather. His mom came in the room and we started talking about my grandfather. Out of no where my boyfriend says "Mom. Do you remember when I would be sitting between grandpa's legs and you told me not to and made me go seat on the seat next to him?" She said yes. He said"Why did you do that?" She said "You know why." He said " Right... My question is, if you couldn't even trust him to let me sit on his lap, why was I around him?" She gets awkward and said "He's still my dad. I love him, but I understand he can't be completely trusted which is why I had to also protect you." He laughed and said "Protect me? Protecting me would be not having me around someone who was a known pedophile." His mom said "It's more complicated than that. I did what I needed to protect you." He is now up and raising his voice. "You did the BARE MINIMUM! And I'm supposed to be grateful for that. Yeah my mother is such a warrior. She did the BARE FUCKING MINIMUM to protect her 6 year old." And he went outside and his mom started crying. I stayed out of it. I felt it wasn't my place. It was so random though. Even his mom didn't understand where this was coming from so suddenly. I just want to know your opinions on him going off on her.