I’m not sure there’s a pattern. I think it’s been theorised that it appears when someone has something to say back at camp, but I’m pretty sure that I’ve seen it when that’s not the case.
Hopefully it will be developed in the full release, if only to provide a story reason to rest and some sort of indication of when the writers think it makes sense to do so. I’m not fussed about having an actual exhaustion mechanic to force rest, though I wouldn’t object to one as long as it wasn’t too onerous. I do think the
D&D RAW mechanic sounds too much, but something like it with the opportunity to avoid exhaustion with a constitution saving throw and removal of all levels of exhaustion on a full long rest wouldn’t be too awful.
