When someone is so fat when they stop walking it takes longer for their stomach and ass to catch up, the equivalent of the 20th ripple after throwing a rock in a pond.
It got its name from the song Mr. Bojangles:
"I knew a man Bojangles and he danced for you ..."
Check out the Bojangles on that guy!
One could also just start singing the song to alert their friends without necessarily pointing out or bringing attention to the fact you're checking out someone's package.