Criteria for Skulls to Appear on Map while Inside Instance

I have been trying to figure out what causes the skulls to appear on the map while actively clearing an instance (not overworld map, the minimap).

It seems like after a certain period of time, or % of revealing the map the skulls appear but I can't figure it out.

I've been keeping an eye on the map to try and figure out when they appear and it doesn't seem to be tied to killing an elite but I could be wrong.

Getting the skulls on the minimap helps a lot with clearing the maps so knowing how/when they'll appear would be really helpful.

Preemptive thanks for any assistance.

Rare elite and boss markers (yellow skulls) appear once there's less than 200 enemies left.

Thank you Froster, that makes sense!