During the "The Thunderlord Clan Approaches" scenario in the Garrison, there are a lot of NPCs which have icons on the minimap which I think are flagged as rare, making the rare alert spam its brains out. There are three different icons that I noticed: an X, a green circle, and a red circle. When you hover over the red circle it says "invader" in the tooltip and "follower" for the green circle. The X is for the Legionnaire dude. Any time a new icon shows up on the minimap the rare alert starts losing it's marbles. The game crashes a lot during this scenario for me so I don't have much more information.
edit: I'm using v5 and no other addons aside from the WoW defaults and Monomyth.