I think Cat knew in her gut that it was a bad look all the way around. She said as much repeatedly. But...the last time she acted on her own instincts and went against Robb (letting Jaime go) it broke down his hold on his camp, so I think she was just walking the line.
I get it but this doesn't explain slipp'in to this disastrous degree. How did Robb ever win a battle again? Geez!
Clearly, they thought it was going to be one way.
That's crazy.. You could sorta understand if it happened at the hands of someone you never crossed, But to not even have a "just in case" against someone you crossed is beyond terrible.