I wonder, have Mongolians ever heard of army maintenance...?
P.S.: They even got GDR's in their ranks: you can see them close to Tabriz and Athens...
So in 2 turns, a roughly 13x17 carpet of AI units managed to fill your entire empire...
If this were possible, you should be chastised for not noticing the 18-100 units visible around your borders prior to the open borders agreement .
Japan did this to me once with about 40 units. Same thing.. had a DoF and those 40 units just stood on my roads and shuffled around a little. When the DoF expired he immediately went to "guarded." I think it has less to do with terrain and more to do with AI shenanigans.
That carpet of doom is pretty unusual though... there's gotta be 150 units in that pic. That's like 600 gpt in maintenance for a human player.