The origin of this phenomena is known. It is likely that such dynamic changes are common in Neptune's cloudtops. The others are much smaller and asymetrical. Strangely, however, Triton orbits retrograde to Neptune's orbit. Such changes are responsible for Triton's geological activity.
None show evidence of recent geologic activity.
If it is prior to 1999, Neptune is the farthest planet. During 1999, Pluto will overtake Neptune and become the most distant planet as it orbits the sun in its highly eliptical orbit. Triton will gradually spiral inwards towards Neptune, eventually passing inside of Neptune's Roche Limt and being torn apart by tidal forces. In approximately one hundred million years, this will result in Neptune having a new set of rings, surpassing those of Saturn.