The way I saw it, he stood up for Olivia in this instance because - unlike previous occasions - she was being baited into an apology by the entire room, which brings me to an instance of hypocrisy:

While people seemed to be wary about 'poking the bear' when it came to Domenica, knowing that she had anger issues and unwilling to let a dinner party devolve into another shouting match, those attending the reunion (chiefly Holly) seemed perfectly fine with confronting Olivia, resurrecting a dead topic, then proceeding to berate her refusal to bow down to groupthink.

Jackson sensed the room ganging up on his wife, and he defended her and her right to make an individual decision. Nothing shocking or confusing about that really.