[jet]

...I think Ryan and Hughie could have planned it all because Ryan was in the bedroom when the scene was shown of Andy and Ed 'discussing engagement rings'. Andy wanted one with a 'solid stone'. When Hughie and Ryan were having their 'joke' they produced a tinfoil ring and and Ryan says he wants a different one when they get out and Hughie says "It's not the ring that matters". It was at that moment I think that Andy caught on because before that he was smiling and relaxed.
But it was just moments after this that someone suggested it was a joke and it became obvious it was. It was then that Andy changed and got up and went into the house. So it could have been either the mockery or the making light of issues dear to his heart that set him off or a mixture of both.