Why would my husband disassociate around his own family?

He didn't used to, but the past couple months I've noticed it happens whenever he's around any of them or even talks to them on the phone. Especially with his parents it happens pretty badly, like to the point i can't snap him out of it like I normally can and I have to either cover for him and do basically all the talking or find an excuse to get him away from them so he can come out of it on his own.

I've asked him about it but he says he doesn't know why that would happen, and isn't even convinced it actually is happening because he doesn't remember it and thinks I'm exaggerating. He usually only disassociates when he's very scared or stressed, so obviously it's concerning he'd start having that reaction to his parents. Nothing between them has changed recently. He still insists on going to Thanksgiving this weekend too, and I'm worried what will happen with all of them together for so long. I don't know what to do about it either though

Why would my husband disassociate around his own family?
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