It makes little sense, as the term itself has as many syllables as the phrase it's meant to abbreviate. Accordingly, it's degenerated into a cutesy term used mostly by girls. It has also been used in the gay community as a candid way of letting others know you are a bottom.
Bruce: Great, I'm a top. See you at my place at 9?
Rolls off the tongue like a bare-backed walrus. Enjoy.
Bt Dubs, N-Dubz run bathtubs.