That's a particular kind of loyalty cascade.
What's happened when I've investigated it is that a group of artifact hunters enter the map together (i.e. announced as entering one after the other: there is no visible grouping of any sort in the UI, although if the group is large you can sometimes have someone who is announced after the last one of the group actually appear on the map before the last few group members. One of the group members then leave the fortress (can happen after a demand to hand over the artifact has been rejected or without a demand ever having been made), then returns sneaking. When the sneaker is detected, he'll either flee or attack (saying different things in the two situations). If he flees and you don't engage him nothing happens. If he flees and you engage him or he attacks, however, the other members of his group immediately turns buggedly secretly hostile (attacking while claiming to e.g. socializing, running around in panic over all the "hostile" locals, or claiming to e.g. socialize while scaring the locals to run around in panic). If you're unlucky, the remaining group members have befriended other visitors or even dorfs, in which case it spills over into a "real" loyalty cascade.
If it's any consolation, the artifact hunters are mercs (and can come back later as such), and mercs have become rather scarce as merc companies seem to have soaked up almost all of them. Also, artifact hunters tended to arrive early (and sometimes in armies: I've had over 50 even after that bug was supposedly fixed), and then either split up into hunting different artifacts or, as mention, become regular mercs.