Sunday night's episode of Game of Thrones featured the show's biggest battle ever, which means, of course, that it ended with a cavalry charge saving the day completely out of nowhere. Yes, just as at the battle of the Blackwater and the battle at the Wall, the battle of the bastards was won by the surprise appearance of the Knights of the Vale, who routed Ramsay Bolton's forces and won the day for the Starks. But only one of the Starks knew they were coming in the first place: A few episodes ago, we saw Sansa send a raven to Littlefinger asking for his assistance in the upcoming fight, a secret that she kept from her bastard brother, Jon Snow. It's not quite clear how much knowledge Sansa had of the Vale army's movements — did she coordinate their arrival on the battlefield with Littlefinger, or did they just happen to show up? — but she didn't even clue any of her allies into the fact that they might have reinforcements coming. Why?
She didn't want Jon to spill.
As the events of battle proved, Jon Snow knows nothing about sticking to a plan. Was Sansa worried that he'd somehow give the game away, in word or deed, if he was let in on the secret? It sounds heartless, but Sansa saw her father Ned's fate sealed by a lax information-security policy. If there's anything Littlefinger has taught her, it's that control of information can be the real power.
She was using Jon as bait.
Speaking of heartless, Sansa knew that the only place to defeat Ramsay was outside the walls of Winterfell. Was she using the Northern army as bait to lure him outside the castle long enough for the knights of the Vale to sweep in from the rear? It's possible, but at the war council Sansa seemed awfully concerned about Jon falling into Ramsay's trap. If he was a sacrifice, why would she care?
She was ashamed.
The first time Sansa encountered Littlefinger this season, she coolly shut down his words of sympathy and nearly had him executed. Then she was forced to backtrack and ask him for help. Is it any wonder she didn't want anyone knowing until the last minute?
She wanted the moment for herself.
All season, Sansa has been chafing with Jon over her role in the Stark coalition: She's nominally just as much in charge as he is, but as the military commander, he's the one with the final say. By not telling Jon about the Vale army, Sansa ensures that the moment of triumph is her moment of triumph, which is important political symbolism if she's going to be the one ruling the North in the future.
The writers needed a surprise.
All of the theories above are plausible, to varying degrees. But often, when discussing things on Game of Thrones that don't quite make sense, it's fruitful to take a Doylist perspective, not a Watsonian one. For the unfamiliar, a Watsonian analysis (as in Dr. Watson) looks at a work of fiction through in-universe logic; a Doylist analysis (as in A. Conan Doyle) looks at it from the perspective of an author. To use just one example, a Watsonian might attempt to explain Arya's unlikely survival this season by formulating complicated fan theories, while a Doylist would just say the show wanted to put Arya in danger but didn't want to actually kill her off.
When it comes to the knights of the Vale's fortuitous arrival, a Doylist explanation says that the Starks need to win, since they're the protagonists, but it can't be easy for them because then the show wouldn't be interesting. Having the victory come from the Vale does a few different things: It makes Sansa's character more complicated, as she's been forced to make a deal with a man she hates to get what she wants, illustrating George R.R. Martin's maxim that the series is about "the human heart in conflict with itself." Having Sansa not tell Jon also adds a potential conflict between Jon and Sansa, and complicates the post-Bolton North in a way that wouldn't be possible if, say, the Northerners had banded together and taken out Ramsay on their own.
Ultimately, though, plot developments should work from both a Doylist and a Watsonian perspective, and that's where this twist fails for me. In fact, it's becoming something of a trend this season: From Dorne to Braavos, Game of Thrones has been making the plot tail wag the character dog all year long. It's not just Arya Stark who needs to keep practicing her needle-work: As Thrones enters its final seasons, the stitches are starting to show more than ever.