My son has been having random screaming fits when my mom watches him. Her and I are the only ones…

My son has been having random screaming fits when my mom watches him. Her and I are the only ones who watch him but for the past month randomly he will start screaming with her. Until he falls asleep usually anywhere from 30 min to an hour

She broke down and cried with him.

I don't know why he does this with her.

It makes me feel horrible and helpless.