It was incredibly risky to claim the throne like he did, but in my opinion it was the last desperate action he could take. He had already taken power with force at St. Alban's but had quickly been ousted once again and back to square one, so for that reason I can see why he claimed the throne, even if it was just what Maragret was waiting for. His claim did have a knock-on effect though for his son, who of course became a very successful monarch and helped stabalise England after crushing the Lancastrians twice at Towton and Tewkesbury.

You're correct Shades, York did blame Somerset for the loss of Normandy and I think part of his huge dislike for Somerset was because he lost a huge tract of land and received, if anything, undue favour from Margaret, who was of course rumoured to have been sleeping with Somerset, although that is most likely Yorkist propaganda. Naturally though, their closeness was pretty blatant, where as when York gained power he tended to be less biased and worked with both sides for the good of England.

I would say it'd be hard to argue against Margaret being one of the biggest factors in starting it all certainly, had she not tried to keep York in the cold so much I doubt he would have ever reacted the way he did, after all when St. Alban's was done he swore an oath of allegiance to Henry again to prove his loyalty was to Henry, not his government. I think the sad thing is that although Henry was weak, he wasn't disliked by the Yorkists at all but his inability to govern those below him who were provoking the Yorkist faction was his downfall. Had he reigned them in following St. Alban's and demanded equal rights to govern for both sides, it is likely the WotR would never have occurred in the first place.