Recently in a large city of mine, 100s of people are apparently moving out. No buildings are dissapearing, just the population. Why? and its not a death wave.
There is a very highly rated (5*) wonderful article / guide for Preventing "Not enough workers" and minimizing Death Waves written by Blake Walsh in steam's Cities Skylines Workshop Guide.
In which it is been specified why people or moving out from the city. The short answer is following but please read the guide for more information on this.
In the guide you can find the following answer in the section starting with "But the most pertinent part of the lifecycle is as follows":
The Young Adults, fed up with living in the attic/basement/closet, decide to move out, if there is no room in the city to start a new family (which is probable because it's choked with old people), they leave the city. This is called the YA Exodus Wave, worker availability plummets, unemployment falls to very low levels and residential demand soars in response to the lack of workers.
To stop this, make sure that you have plenty of space for them to move into, but most of all, build part of your city then wait for some time before building the next so death waves are much easier to handle. This ensures that people die at a maintainable rate, so people move from house to house until they die. This is the most efficient way to solve this problem if you want to avoid downloading mods.