"Wind is the most likely explanation for the plume distortions you refer to.  As I understand it Triton has polar caps that might change with the seasons (although there's some speculation about this) and the southern hemisphere was some ways into spring when Voyager 2 flew past.  What may be happening is that frost that has formed during winter vaporizes during spring, increasing the local atmospheric pressure.  This tends to transport vapor away from that region to the opposite colder hemisphere as the 165-year long Tritonian 'year' progresses, so as one cap shrinks its cousin on the opposite pole might grow and vice versa.  It's this migration of vapor from one hemisphere to the other that is believed to be the source of Triton's 10 meters per sec or so wind.

   "As to why the plume rises to a particular height where it appears to be sheared off, if I recall correctly this is likely due to a combination of the force with which the material is erupted (if we're talking geysers) and buoyancy.  The material in the plume is probably somewhat warmer than the surrounding atmosphere, and at a certain height the temperature of the atmosphere reverses - so the plume stops rising.  That's where the wind comes in and shears the plume off in that right-angled pattern, taking the material downwind and causing the streaks."

"The Surface of Triton"  By Don Dixon