If this is happening, the major reason for it is the fact that some white Americans are coming to believe that their dominant position in American society is under attack.
When white Americans were completely dominant in our society, they did not need to feel that they were a racial group. It was not necessary for them to feel this because they were simply the most important group in society. They did not get treated in some generic way because of their race and therefore they did not need to see themselves in racial terms.
But now, many whites think that they are losing their dominant place in our society. All of a sudden, they have come to feel that they have interests that are based on their race. Now that they feel that they are threatened, they feel the need to come together as a race so as to defend those interests. In this way, the perceived decline in the status of whites has led to (one can argue) the rise of whiteness as a racial identity.