A pellet in flight is subjected to two significant external forces. Gravity which pulls it down towards the earth, and wind resistance, aka drag, for which the term BC or ballistic coefficient is widely used.
Force = mass x acceleration. As in our case the mass of the pellet is defined, and doesn't change in flight, then these forces are accelerations. Gravity is about 32 feet per second per second (in the old units!). Drag is a negative acceleration, in that it slows the pellet down, and the higher the drag the quicker the pellet loses speed. In terms of magnitude, the deceleration due to drag outweighs gravity usually about 25 or 30 to 1.
Chairgun will tell you a 7.9 grain JSB drops from 810 fps to about 610 fps at 55 yards, and takes about 0.24 sec to get there. That is a change of 200 fps in 0.24 sec, something over 800 feet per second per second.
Whereas gravity is pretty well constant, wind resistance can vary a bit with the density of the air and its viscosity. Yes, air does have viscosity, and perversely it gets more viscous as it gets warmer, which is counter-intuitive. But, for the distance of the flight path, that air is not going to vary very much. So the deceleration due to drag is also reasonably taken as a constant.
For a pellet to suddenly start to slow down more quickly, the drag factor must have changed in flight.