Might have found the main cause, might have found some other supplemental causes.
Is it all guests and all slaves effected, or just guests? Also, are only females effected? And what was the happiness threshold you set for Day Off? Red, yellow, light green, or dark green?
Regardless of the threshold, if the answers to the eother two questions were "all guests, regardless of gender", all of the following might be relevant.
Some interesting bits I've found so far:
First, here's the code that defines what mood an NPC has, thus what smiley face they are assigned in menus:
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Next, a bit of code that causes all guests mood to drop should one decide to leave. I think it would mean that if someone leaves, every guest loses 80 or 50 mood depending on their relationship with the leaving NPC.
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A guest will choose to leave if the day ends when their relationship to the MC is under 0 or thier mood is -70 or lower.
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There also appears to be some code to cause all guests to lose 5 happiness if there are fewer than three spare beds.
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I suspect that your bunkhouse being "overcrowded" or not having an overabundance of food, mixed with weather events and possibly other stuff I haven't found yet, caused multiple NPCs to have mood drops over multiple days. Once one finally left, the "overcrowded" penalty wasn't applying anymore, but them leaving caused everybody else's moods to drop again. So another followed due to bad mood. Then another. Then the mass exodus began.
As a brief aside, though, I suspect this starting around the time Blair mentions automation has less to do with automation being unlocked and more due to the fact that it took a while to get enough bunks/people for this cascade to trigger. A -5 to mood for everybody is a minor thing when you only have 3 or 4 people since, due to random interactions, you can gain that back pretty easily. Once you have 25 or 30, a random encounter with any individual character become less likely.
That said, I wish there was some in-game indication that overcrowding or food scarcity is causing mood loss. So far, it seems like there are a lot of elements working in the background against you with no rhyme or reason explained. Probably still some that I've missed.
