On a purely language evolution standpoint, isolation and a bias toward how things are spoken with a particular accent.

As for sudden changes within a language, probably someone uses it in a derisive manner at first, and it becomes more commonly used. When at last people do use it more frequently, most do not actually intend to use it in a pejorative manner, but as to describe something else. Gay would be a good example, as it went from:

Happy --> Homosexual --> Lame (though it is still used concurrently with the previous definition)

Or perhaps a change in who uses it. The N-word is another good example in the US. At first it was black-black, generally more senior slaves talking down to newbies. Then it went to white-black, where masters would scold. Afterward, when this word was left alone, it represented hate and malice. Now it is once again used black-black (God forbid whites or Asians use it), in a jovial or delightful manner. At this point, it is now a more crude, black-only form of the second person.

Wow does this post sound 100% racist and borderline uninformed. Excellence at its finest.