The reason the Starks let their guard down is because of the hospitality custom. In the beginning of the episode you see the Starks eating bread and salt. That's the beginning of the custom. Once you eat bread and salt from another man's house. You're brought under his protection and are supposed to be safe. That's what Walder Frey was talking about in the beginning. Frey broke something that was almost as sacred as the bedding after a wedding in Northern tradition. It's like you can break every tradition in the book but the hospitality one was supposed to be sacred.
^^^This. Rob knew he was walking into a dangerous situation. The show director made sure to spend all that camera time watching the salt and bread get passed around, hearing Frey say he was under their protection, and having King Rob make that long ass apology for a reason.
The goal was to get both Rob and YOU (the watcher) to let their guard down.