What determines battle order?

[Endoperez]

The game does not keep book of the previous owners of provinces. Previous owners have no effect whatsoever in the game.

And for armies moving towards each other, the bigger army has a better change to push the enemy back to the province from which he left. It seems you just got unlucky. If both armies would be small, I think they could also miss each other and both would try to conquer the province previously defended by that other army.