Well if you have poor public services, such as a Health Index lower than say 80. High unemployment may also cause people to just leave. But typically where you find population decline is in older, more populated nations where they have a high Health Index, but this causes the population to have a high life expectancy. This causes the population to age and eventually die off, and since there aren't much people in their 20s and 30s left in your country, you are unable to replenish your dying old with newborns.
For disasters, as long as you're prepared an have food units and inoculation units, for the most part you'll be fine.