As far as I can tell, there are just random servers that I can't connect to, and those manifest as groundhog errors. If I get groundhog while in a fireteam, I can't rejoin the team until they move to a new area/cross a loading zone or hit orbit. I most often see the error when trying to join missions or cross loading zones. I'd imagine that when it happens mid-mission, it's some kind of host migration.