[Chazar]

I don't think the "interecepting armies" thread helps here much: the described behaviour should not occur, i.e. armies orginating from the same province should not split up when ordered to move together.

However, it is likely that the OP missed something, like magical movement, assassination, stealth, miss-clicking (like unequipping/moving an item without noticing that this reset the movement order, etc.), and so son.