There are some bugs associated with settlements that cause the happiness to crash to low levels. I made a previous thread about it, but am unable to edit the thread title, so I am making a new thread with a title that reflects that I have a consistent workaround to address the bugs.

First time linking in these forums, so I may have to tinker a bit in the editor after the fact:

https://www.youtube.com/watch?v=yt3Fd-MPBzQ

[youtube]https://www.youtube.com/watch?v=yt3Fd-MPBzQ[/youtube]

The video uses in game footage to explain some of what is happening when people are experiencing unexplained happiness drops in their settlements. It also demonstrates the workaround.

The issue is caused, in part, by a bug where a settlement's assets register incorrectly, as explained in http://www.gamesas.com/topic/1568732-settlements-constantly-revert-to-oldbroken-snapshot/#entry24669430.

The bug is worsened by multiple construction phases on the same settlement, but tends to stabilize over time if no further construction changes are made. The symptoms express themselves when the player fast-travels from the settlement. Checking the pipboy will show incorrect lower values in the beds, food, water, power and/or defense categories. Returning to the settlement restores the values to the correct numbers. To workaround the issue, run some distance from the settlement before fast-traveling. You should run at least far enough that you no longer register as being in the settlement. If you do so, when you arrive at your destination, check your data tab in the pipboy to see the numbers for your settlement, they should be correct. This is temporary, and eventually the problem will re-express itself, but much, much less often if you consistently use this workaround. The happiness will stabilize over time if you are consistent in leaving your settlements on foot before fast traveling.

Be sure to periodically check your pipboy for signs that a settlement is experiencing the bug - the incorrect numbers listed should be fairly obvious when it is happening. Return to the settlement as soon as possible to set the correct numbers, then leave on foot for some distance before fast traveling away. Check the pipboy again to make sure it worked, and reload if it did not. Try running further before fast traveling the second time. If the happiness has stabilized, you have a buffer of time to address the issue if it arises while you are in the middle of a mission. You can complete the mission and return, as it will take several days before the happiness drops to negative levels. Try and avoid leaving the problem for several "in-game" days in a row.