According to this source from SOA/NASA Astrophysics Data System on Tidal evolution in the Neptune-Triton system (Chyba, C. F., Jankowski, D. G., & Nicholson, P. D.),
Transcript: Many investigators have speculated on the reason for
Triton's retrograde orbit. Lyttleton (1936) suggested that both Pluto
and Triton originated as prograde satellites of Neptune, only to
experience a catastrophic gravitational interaction. McCord (1966)
showed that tidal evolution could have brought Triton inward from a
near-parabolic orbit, lending plausibility to a capture origin for the
satellite. Jarrington and Van Flandern (1979) suggested Triton's
peculiar orbit and Pluto's supposed "escape" were caused by ab
encounter with a massive "rogue" body, a conclusion contested ny
Farinella et al. (1980). Finally, McKinnon (1984) combined momentum
and energy constraints with an improved knowledge of Pluto's mass to
show that all Pluto-Triton interaction scenarios for the origin of
Triton's retrograde orbit are untenable, leaving the capture
hypothesis as the most likely possibility.
Now, that summarizes all the theories one might have about it, at least all I could find.
As far as evolution of the system is concerned, the same document follows up to it and demonstrates how tidal effects are responsible for the evolution of the system. "Degrading" is a strange term. Let us go with evolution.
One of the possiblities stated is that Triton will reach a perfectly retrograde state and stay in it for ever! For a detailed analysis, you may read up section 6 in the link provided, "The future of Triton"