Chances are, you are seeing a difference in the way happiness is calculated. Because the formula has been changed, I have no idea how it works now. In a few weeks, the dataminers and testers will have mostly figured it out, but until then, any suggestions by myself or anyone else are going to be guesswork.
Unless you continue to experience a downward trend noticeably past 80, I wouldn't get concerned. Most likely it's just a graduated adjustment to whatever your current stabilized happiness is.
But, just in case, here is what I can think of to check.
- Check your pipboy while you are away from the settlement. Make sure all the numbers are correct. If some of them seem to be off, then you are most likely experiencing the fast travel bug that incorrectly lists values for assets and comes with negative happiness. I know reports that it continues to happen since the patch have come in, so I believe it remains a possible explanation. While you are in Sanctuary, the numbers are likely to be correct, so you won't know if that's the problem if you're actually at the settlement.
- You may have recently experienced an attack that you failed to defend. Some people have reported a temporary negative penalty to happiness from a recent attack. I haven't seen that myself (although I have seen a secondary symptom of happiness going down as a consequence of assets lost in the attack.)
- A recent change to your settlement's population or assignments may have resulted in a happiness recalculation without you realizing it.
- Settlers can occasionally become "unassigned" from a task. This usually happens if they are a new settler who has been assigned to something for the first time. Unassigned settlers can impose a happiness penalty, so you may find someone not performing tasks, and discover that they don't have one assigned.