Just like in breach, whenever it's Samurai map, the defenders are Samurai. Whenever it's a viking map, the defenders are vikings and same for the knights.
Make it same in dominion, e.g. Defenders on the Sanctury Bridge will always be samurai minions, Defenders on Sentinel will always be Knights so it makes sense, It's just a little detail but some people who pay attention to every detail in the game (from everything apart balance) would really appreciate it including myself.
I think this is actually a very intentional design choice. The maps vary in locations depending on which Faction holds a particular territory in the Faction War, or at least it's supposed to give that impression.
We're fighting a back and forth war, and the maps represent keeps and strongholds being lost and reconquered frequently, hence the change in who defends it. If you examine the maps closer, you'll see that apart from the minions changing, the backdrops and decorations adorning each map change according to which Faction currently holds it.
If the Vikings hold a map for instance, there'll be runic banners and painted shields adorning the walls and ramparts. When Knights hold a map, they'll be flying more traditional heraldic banners. The effect is actually pretty cool on overtly faction toned maps, like Citadel Gate. The map still looks distinctly Knight, but when Vikings are defending, seeing their banners fly gives it a nice contrast and a vague sense of immersion.
For Honor's been skimping on lore and immersion since Season 1, so the little things like this I actually welcome.
Guess it would be way better if you simply fight with your own minions everytime. Even if Dominiom/Breach is at a foreign war front you always see your own minions at your side.
If there is a multiculti group, then everybody is shown the minions of his own faction.
Please make it happen for immersion sake!