The part he's actually talking about was that one guy was being a massive dick and refusing to dance with the Mormont girl. I think she was being pretty forward with him and since he was secretly wearing chainmail underneath his clothes he didn't want to get too close with her, so he stormed out (or tried to). Cat went over to him because she sensed something odd was going on and went to grab him as he was leaving, and she felt that he was wearing armor on, which immediately made her realize that something was up. . . since normally you don't have your guys wear chainmail to a wedding.
Basically, once she realized they some of Frey's guys were armored she knew they had been betrayed.
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