I was watching the jcsat-14 launch, and I noticed the vehicle was slowly slowing down when 2nd stage engine was off even at 250+km. Is it because of atmospheric drag or pulling force of gravity?
Thats because when the burn ended, the stage was near the lowest point (perigee) of its very elliptical orbit
and going "up". So gravity is slowing it down until it reaches the highest point (apogee). Then it will start gaining speed again as it "falls" closer to Earth.
For elliptical orbits the speed is not constant, but it is lowest at the highest point and highest near the lowest point.
Imagine sending a ball rolling uphill - it will slow down while going up and then when it goes over the top, it accelerates again.