The origins of "Talk like a Pirate Day" is derived from the "Time for Timer" 1970's series of PSA cartoons. One specifically that used a partial out of the ink well story formula about two kids and their fat, greasy, unkempt uncle (possibly a father) who cut his finger or hand, and was a special as it was either 30 minutes or almost an hour in length, as opposed to the 30-second PSA spots that aired during commercial breaks.
That famed American animated Saturday Morning "celeb", Timer, who reminded kids not to drown their salad with dressing, or to eat fruit because it was "nature's candy", took the two children into their patriarch's innards, and explained how various systems of the body worked, including how his immune system were fighting the germs infecting his cut.
It was an anatomy lesson.
So what does this have to do with "Talk like a Pirate Day"?
In this Timer special the characters (Timer and the two kids) come across a pirate captain who states "Arr, matey, walk the plank, and other pirate like phrases..." As he commands his pirate ship inside the uncle / father's blood stream (fighting white blood cells). This idea was held onto by one of PBS's / NPR's avid viewers / listeners, and found its way to the NPR powers that be, who decided it would be a fun idea to pay homage to various Hollywood iterations of pirates.
That's my story, and I'm sticking to it
EDIT;
Said piece was "The Incredible, Indelible, Magical Physical, Mystery Trip"
http://www.imdb.com/title/tt0322745/combined