There are a number of interesting articles already posted onto RG. Here is the link to one of them-

https://www.researchgate.net/publication/233403526_Review_article_Liver_transplantation_for_the_pulmonary_disorders_of_portal_hypertension

Article Review article: Liver transplantation for the pulmonary diso...

It is not just dilation. there is also aberrant vasculogenesis in HPS; probably VEGF driven. The basic connection seems to be an overflow of the factors driving splanchnic vasculogenesis to reduce portal hypertension, into the pulmonary system leading to undesired aberrant vessel formation and dilation of existing vessels. This is seen in experimental models.

Anoher cause implicated is the bacterial translocation , activate NO , VEFG, ET1 and more. Reciently  we have a patient who died 2 month later the diagnosis. Very bad prognosis. 

This is an article  WJG , very usefull