Castle Reinforcement

[Mysterio]

For all you experts out there, I need an explanation:

My Army A is in my castle that's currently under siege and the walls have been broken down; the enemy can storm the castle next turn. I want my Army B to join Army A behind the castle walls, since I anticipate that the enemy will storm the castle. So, I order Army B via the "Move" command to move into the province with that castle. Why did Army B instead engage the force that's besieging my castle when I didn't give them the "Move and Patrol" order?

Thanks.