I think a fair number of them just no longer have any fight in them on some issues, particularly the amnesty for illegal aliens issue. They just want to get rid of the issue so they won't have to deal with it anymore. Very short-term thinking to make it easier on themselves with little thought to the longer term result most anyone is able to see: millions more votes for the Dims and few more for the Republicans.
Some Republicans are just yelling: No mas! No Mas! on the amnesty for illegals issue, and caving in is the only thing they know how to do at that point.