Someone suggested in an old thread that perhaps Shagnasty was the skinwalker that Morgan nuked. He survived it, and over time was able to reform. However, it would provide a strong reason for him to be pissed off at Morgan.
It was also suggested that Shagnasty's attack on Raith manor was to help Madelaine escape. That is, she heard about Harry and Luccio visiting, and made a phone call asking for help.
APG