Big star, big planet, big orbit... Would that be surprising? The HAT-P-8b discovery paper says that planet challenges the hypothesized mass-period relationship, so it may just be due to the small sample size. The explanation for some of the observations in the paper you refer to might have been found in How eccentric orbital solutions can hide planetary systems in 2:1 resonant orbits by Anglada-Escude et al. The apparent lower eccentricity of multi-planet systems is what you would expect if there are multi-planet systems hiding behind single planet solutions, as Anglada-Escude et al. suggest. This possibility is well illustrated by their fig.2 which I have taken the liberty of reproducing here.



This mistaking of multi-planet systems for single planet systems could also contribute to the apparently greater eccentricity of more massive planets.