Why is the eccentricity of Triton so low? Triton is very special, since its orbit is retrograde respect Neptune, its obliquity is near 0° and its orbit is almost circular (according to Harris (1984) is $5 \times 10^{-4}$). I'm familiar with the theory that claims that Triton was several Myr ago a KBO that was captured by the gravity of Neptune, but I didn't find an actual explanation of why the orbit is almost circular. Can anybody give me some theories that actually explain the cause of this? I've heard about tidal circularization, but I don't know if that explains it at all...
Tidal circularization is indeed a plausible mechanism for why Triton's orbital eccentricity is so low, and I believe it is in fact the prevailing theory, at the moment. In short, tidal forces from Neptune dissipate energy in Triton's interior, essentially "squeezing" the moon. This results in a loss of orbital kinetic energy. The exact calculations that lead to the decrease in eccentricity are a little complicated - and described in the paper you linked to - but in a nutshell, Triton's orbit could have circularized on a timescale of about $\sim10^7$ to $\sim10^8$ years (Ross &amp; Schubert (1990), for instance, computed the latter order-of-magnitude timescale).

Tidal circularization is not the only possible explanation for Triton's low eccentricity. Cuk &amp; Gladman (2005) hypothesized that there was once a debris disk around Neptune, possibly caused by the collision of two or more other satellites after gravitational interactions with Triton. Interactions with the disk then contributed to energy loss from the moon in the same way that tidal circularization might have, thus leading to a smaller eccentricity. Interestingly enough, though, this could have happened within only $\sim10^5$ years! The authors have suggested that a hybrid of the two models may be the best explanation.