So when did...

The Doobie Brothers and Eagles become country groups?

During the hight of their popularity in the mid to late seventies, I seem to remember (I was 10-15 then) that they both were considered a Rock band. Just now, my iPod brought up a song off of a collection of Doobie Brothers hits which in 2007 sounds more Country than Rock. I recall the same effect happened to the Eagles.

Obviously, the music hasn't changed. So when did musical styles change so that what 25-30 years ago was "Rock" is now "Country"