I'm not an expert on the art of raiding but here's some stuff I just thought up
-Player tends to make improvements faster than AI lords so their fiefs tend to be wealthier, which makes it a prime target for anyone who's trying to raid.
-Lords who don't like the player will raid his fiefs just because they don't like him, they also tend to have ****tier personalities which makes them more prone to act alone and not follow the big army.
-When the player first oaths in the king will give him one of his own villages or any village that's left unlorded, these fiefs are often contended territories close to the borders so it's right there in front of the enemy a lot of the time.