New Player Question --How am I Getting Flanked?

[Redfive]

I'm into my first pretty serious game as Ermor and have been progressing well until I was mysteriously flanked by Pythium. Specifically, I had a two province front with Pythium with water on either side and significant troops/defense in both. Out of the blue Pythium takes the two provinces BEHIND the front completely going around my defenses.

As far as I can tell, they don't control anything adjacent to these two provinces as they are bordered by yet another of my lands to the east and a neutral water province to the west. Is this some game mechanic I'm not aware of? How are they able to simply bypass my main forces and attack into non-adjacent territory?