Political Correctness

Why has the meaning of political correctness changed so much over the years? Being PC used to mean not calling black people n-words and not call wemon dumb. Now being "PC" just means pretending to be offended whenever someone says one (of the many many words) that are deemed incorrect. Not to long ago Jennifer Aniston called herself a retard on TV and everyone in there laughed over it. Latter that day the media had already started in on the "OMG SHE SAID WHUT" crap. When and why did it change from a usefull standard for how to speak to others into pretending to be offended at everysingle thing you can think of?