I suspect Pat's behaviour is part of the 'it'll get worse before it gets better' scenario. And it's not entirely implausible when you think about what the poor woman's been put through in the last few years. There's been nearly losing the dairy business because of the contamination of the yogurts (or was it the ice cream?), finding Johnny and being told they couldn't see him before it all came right, Helen's anorexia, Helen and Tony not speaking for months when she was pregnant with Henry, Helen nearly dying giving birth, Tom leaving Kirsty at the altar, Tom running away to Canada leaving his business in a mess and nearly losing them their organic status, Tony's accident ... It's a wonder the wretched woman is still upright. This can fairly easily be seen as the last straw.
I smell a plot device, part of which was to give Peggy her moment - and btw the fact that she talked sense to Helen this time doesn't mean she's suddenly not sanctimonious, judgemental and narrow minded! I can imagine every member of the family having their moment to shine over the next few months and I wouldn't be surprised if Pat's is in court.