Hoppy, weather conditions control how fast gasses and smoke trails disperse. You've seen fast moving and changing clouds at times and unmoving unchanging ones at other times, right? Also, depending on the height gasses tend to disperse differently in the troposphere than the stratosphere or lower mesosphere.
Edit: Having now, skimmed your video, I should also note that the differences in trails can be a result of gearing up the engines increasing the amount of water vapor condensing from the exhaust, or by changes in air pressure. This easily accounts for the on/off switch your video assumes as well as differences between two planes in different areas of the sky.