So here’s what I’ve been able to piece together based on wild internet theories: Pat’s an abrasive piece of shit who managed to finally rub Matt the wrong way one too many times. That’s likely what happened with him and Liam, as well, since Liam now wants absolutely nothing to do with Pat. Only Woolie will still have anything to do with Pat, because everybody loves Woolie, because Woolie’s easy to get along with.
Of course, we all knew that Pat was abrasive and caustic. That’s one of the reasons we like the guy; he has no filter. Matt and Liam are far more sensitive personality types, and they have a much lower boiling point, so to speak. Now there’s this awkward broken quadrangle friendship going on where Pat will talk to Woolie, but not to Matt or Liam, Matt will talk to Woolie and Liam but not Pat, and Liam will talk to Woolie and Matt but not Pat. And Woolie will talk with everyone. It’s all very awkward and will be at anyone’s future wedding.