The answer to this question can be found at the beginning of Chapter Two, when Hannah and her family arrive at her relative's house to celebrate Passover. As Hannah enters the living room, she is treated to the spectacle of her Grandpa Will shouting at the television screen, saying "Give them this!" as he brandishes the arm that bears his tattoo on it towards the images that are presented to them. We are told what was on the screen:

Across the screen marched old photos of Nazi concentration camp victims, corpses stacked like cordwood, and dead-eyed survivors. As the horrible pictures flashed by, a dark voice announced the roll of camps: "Auschwitz, Bergen-Belsen, Chelmno, Dachau..."

Having such visual reminders of such a painful period of your life undoubtedly is going to cause great grief and upset, which explains Grandpa Will's anger and strange behaviour as he is forced to relive memories that he is at once unable and unwilling to forget.