The Social Security Trust Fund, the special government account from which Social Security payments are made, is projected to experience a shortfall in the future because

Because of demographic changes.

Explanation:

Our society is experiencing a severe demographic change, which will directly affect social security, causing an increasing deficit.

This demographic change is happening because the birth rate in the country is getting smaller, moreover, with the expectation of life expectancy getting higher, we can conclude that the country has more old people than new people and this will get more evident over the years. However, this trend decreases the number of workers, able to pay social security taxes, but social security will be increasingly necessary. As a result, there will be a lot of money going out and little money going in, which will generate the deficit.