I can't support the authenticity of it, but the explanation given to me many years ago was that any song can be played as a "breakdown," in which the normal "dance pace" is abandoned in order to show off the virtuosity of the musicians. A few tunes that were easy to play at "breakdown speed," or were frequently played that way, picked up the "breakdown" in the customary titles.

It was also - by the same sources - claimed that the musicians would play pieces late in the evening at increasingly rapid pace - speeding up during each piece - to wear out the dancers so they'd let the band go home.

Since bluegrass became popular, they seem all to be played that way, so one might question the need for the term - if there's anything to that explanation (which there may not be).

John