Forum:The siege of Winterfell

Hello,?

I have a question concerning the siege of Winterfell, and I hope you guys can help. I don't understand exactly why, at the end of the 20th episode, Bran, Rickon, Osha and Hodor find? Winterfell on fire. The city was under siege by a Northern army, allied with the Starks, so I can't? imagine them setting the place on fire. And the 20 soldiers who betrayed Theon dont really have a motivation for burning the place down, set aside the practical objections for 20 people burning a city of this size. So is there anyone who can explain what precisely happend? there? Thanks a lot!? ?