Some beginner gameplay and strategies questions

[Zywack]

Question about something that just happened in the multiplayer game I'm part of. earcaraxe did suggest something but I'd like to make sure I understand it correctly since one part doesn't click in my mind and I'd really like to understand this so that I can prevent this from happening again.

Let's say I have province A and B, the enemy has province C (with a castle) and the other provinces are neutral.

I give the order to A to attack C. I give the order to B to attack C.

What ended up happening is that A attacked C alone, while B didn't move: Their order simply returned to "Defending". They didn't get attacked or had any event happening. I double-checked the turn I sent, and I did have all my movements configured correctly.

How can this happen?

The enemy caught a scout of mine before the battle and their entire army fought scout which indicates the enemy was likely on Patrol. Plus, during the attack from army A, their entire army was defending outside the castle, which should indicate that they had all their forces on Patrol. If I understand the patrol/castle defense mechanism correctly at least.

The only way that I could sort of see is if they had given orders to attack B. My army at B consider itself under attack, so it switch to defending. My army from A lands on C, stopping their movement and making them defend. Is that correct? It doesn't make too much sense, but it's the most logical explanation I can think of right now...