The situation is a Crylithium Curtain Wall right in front of a Ballista Tower. For whatever reason spectres are much more likely to attack walls than other creatures (possibly because they walk to slow?). The monster is attacking the wall right in front of the tower and for a short period the tower is attacking it. Then suddenly far away a Drone enemy appears in range of the tower. The tower then refocuses on the Drone and keeps attacking Drones while the spectre pounds away at the wall right in front of it, until the wall is destroyed. Enemies who are actively attacking buildings/walls should be prioritized I believe. Towers seem to prioritize Drones over all other creatures as well (is it alphabetical ordering?).
There have been 6 items by mlindner (Search limited from 28-November 19)