Why is the eccentricity of Triton so low?
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.