At last! I have successfully simulated what the AI does that makes it look like they're secretly in alliance together with monsters!

Here here! Let you all look at this picture so that you can see how I did it!

I was screwing around cuz I didn't like my starting spot, too close to challenging difficulty AIs and their cheats but I wanted to explore so I explored.

my Bannon on return trip to capital was at the green circle just west of the ruined keep. When I pressed end turn the tribe of darklings moved out of their encampment in blue circle to kill Bannon so they ended their turn north of the ruined keep.

This exposed their encampment to pillaging by my Bannon and pillaged with all my might I did!

Then I pressed end turn and I noticed my bannon lit up and darklings was just a wee bit too slow in attacking me so I ordered Bannon to move two tiles east. And bam, the bannon moved at same time as darkling army tried to catch bannon but failed! SUCCESSFUL PILLAGING AND ESCAPE!

Thus Simultaneous movement is apparently very advantageous for the AIs to exploit easy looting due to them being able to move fast. Humans can do it too but they have to pay attention to the unit they want to ninja with and do it very fast. And they only can do it with one unit a turn for obvious reasons

Oh and my Ericridge is injured because he tried to raze the AIs starting village surrounded by four monster armies but failed cuz he got caught and killed welp. Restart! I got rekt.