Why does Neptune only have one major moon?
It seems possible that Neptune once had a system of moons similar to other outer planets. However Triton, a Kuiper belt object was captured by Neptune, and this disrupted the existing moons. It would have initially had an eccentric orbit, intersecting with the orbits of other moons, setting them into unstable orbits and ultimately either colliding with each other, hitting the planet or being ejected. 

The minor moons formed from the debris of these collisions after Triton had entered a more stable orbit. So the disruption was the capture of Triton.