He heard Mary say it loud and clear, she heard his comatose brother's mysterious fiancée say she was pregnant, another piece of good and sudden news for the family, but for him it was like being hit below the belt.
Jack acted on his own, he just wanted to laugh hysterically at how ridiculous it all sounded, but instead he just followed her, trying to ask if it was true or not, ending up at a Christmas party with her, where his paternal instinct or whatever it was set off all the alarms when he saw her drinking the strongest drink he had ever tasted, immediately trying to stop her.
She looked at him as if he were the one committing an act against health and the only solution was to state the obvious, according to Mary: “Because it's bad for the baby.”
But the music stopped a few seconds before and when all he got from her was an offended silence, he knew he had said the wrong thing, besides having shouted it in front of people who were strangers to him but very close to her.