F-16 low speed SAM avoidance tactics?
The missile works by predicting where the airplane will be. If you maneuver, it has to re-adjust, wasting energy. 

When the missile has lost enough energy, it'll eventually fail to re-adjust. 

Now you can try to beat the missile by being far away and fast 
or
you can try to make the missile lose a lot of his energy by doing turns, at low speed. (You can even nodge the missile into the ground by making some close-range, low altitude dives)

In this instance, the pilot was probably already low. Or needed more energy for maneuvering and traded his altitude for energy by descending.