I'm guessing trolls attacking bandits in this area if they get close enough is intended, but what happened to me seems strange. I attacked one of the 4 trolls in the upper-left corner of the map and then run away from them so only one would follow me. For whatever reason the others went straight to the bandits camp and started to attack them even though I was nowhere near close to them (I only realized what was happening after seeing the combat dialog).

 

 

They didn't saw the bandit they normally send to explore on the left by the way, they attacked from the South.

That the trolls don't see the bandits as allies is normal. It is the same for other groups of monsters. One cannot rely on that always ... but can abuse it a few times and trick two groups into fighting eachother.

 

There are some bugs in 3.0x so far ... where units sense enemies over a very long range and start combat mode. ... At Crägholdt I've been stuck in a neverending loop of combat mode ending only to start again the next second. Almost made me mad. Similar to the Galawain's Maw problem ... so I explored the map while combat mode was restarting every few seconds. No enemies nearby. All slain. Only a single enemy had left the battle field and had moved far to the north to attack monsters there. It took some time before I found him. Finally, combat ended without restarting again.