Question

Why is the expansion slowing down in most big bang models?

Answer 1

Because astronomers can find no reason for it to continue expanding.

Explanation:

For the universe to be expanding either at a steady rate of accelerating rate, they need to find something providing the energy to support such states. They believe the universe is expanding at an accelerating rate and have used the idea of "dark energy" to explain such a thing.

Answer 2

The current big bang models contain factors "Dark Energy" and do not have the expansion slowing but accelerating

Explanation:

Here is a link to a video that says that "Dark Energy" took over about 5 billion years ago