These Spire mobs also appear to randomly pop up in Sunderock at the callers area. All the callers disappear, the zone shakes, and the mob pops out for 2minutes with a 'who woke me from my slumper' zonewide shout. Mob can't be attacked, doesn't respond to hails, then disappears.
Perhaps the reason they can't be attacked in Sunderrock when they spawn like you describe is that all the mobs around their spire weren't dead at the time. Perhaps if they were you could kill them?