I don't think there's a bunch of background there. Maybe Clay would have a beef with Unser because he and Gemma are close, but I doubt he had anything to do with that attack/home invasion. If it was one of the nomads, maybe they joined SAMCRO for other reasons. :shrug
I can't get that scene with Tig and Pope out of my head. Just gut-wrenching. I rarely cry when watching this show, but that totally got me. It wasn't because Dawn was a beloved character - it was because Kim Coates was just incredible. Nice interview with him on TVLine about the episode:
‘Sons of Anarchy’ Season 5 Premiere — Recap and Spoilers - TVLine