It was coming in for a landing at Ontario, California, and its altitude was dropping, as recorded in the FlightAware.com logs. Nobody is assuming anything.
The contrail appeared thicker 200 miles away because the winds aloft were about 80mph that day based on images from the GOES weather satellite, and they'd had about 20 minutes to spread out the distant part of the contrail.
If you look at a contrail from an actual missile launch, the difference between surface winds and winds aloft causes the upper portion of the contrail to spread differently than the lower portion.
That's not what the logs say today. See post #259. But it is now being said that it wasn't an altitude change that caused the cessation of the contrail it was because it was getting closer to land. The contrail explanations change regularly so you have to keep up.