Swearing In

What is the deal with the Rupublicans refusing to swear in Bush cronies when they testify before congressional committees? When they questioned Gonzales about the wire-tapping scandal, the republican head of the committee refused to swear him in yet again. This also happened with the oil executives when they were asked to explain their record profits and it seems to be becoming a trend. Are these republicans scared to have their friends caught up in lies? Is this a common occurrence that has been happening for years that I don't know about?