The bVII or dominant bVII7 chord often comes from the mixolydian mode. Many bluegrass and rock and roll songs are written in the mixolydian and not in the major mode (or ionian mode, or major key, or major scale).
In the key of C, the mixolydian scale is
C D E F G A Bb C.
So the chord is built on the note Bb in this mode.
Since there is only one note different between the major key and the mixolydian, many people will hear a song in the mixolydian and mistakenly think that it is in the major key.
You probably know a great deal of songs in bluegrass, country and rock that have a melody in the mixolydian mode; you just never thought about it.
Here is a link to an analysis of "Sweet Home Alabama" that explains the mixolydian mode an the use of the bVII chord in this context.