After capturing all the Paeonia cities I discovered masses of Paeonian units between the NW Paeonia Watchtower and the N Penestae Watchtower. They had all ran out of food so were very easy to capture. I ended up with over 300 slaves.
Why did the Paeonians hide all these units and not use them when I attacked their cities?
As I tried to get the slaves moved to cities or mines one unit of slaves who hadn't moved started spinning around. Why?

Sometimes multiple AIs get into some wierd "fight" where no one dies, but everyone runs out of food.

When you appear, they fight for real and everyone routs everyone else....

I've seen that too, actually very close to where Anon described it, yet in my case it was an substantial Illyrian force, mixed hoplites and peltasts, all stationary and starving, routing upon first contact with my troops. This was after I had taken all their southern cities, with battles large enough to be fun and challenging - but it would have been more so if these other units had been involved.

Pity they didn't join the fight to save their kingdom but you got the manpower to work all those mines in Paeonia and beyond. Didn't have the same issue when I conquered Paeonia. Beseiged 2 cities at a time and the garrison in each city all came out and fought my besiegers everytime. Had to make sure my catapaults were behind some hoplites/phalangites and not just peltasts/javelineers. Those tribal cavaly units are tough on light troops. Captured all routed units to work the many mines in the area.