The wiring goes bad, and that will actually kill the tranny...you can't access it on the car, so no one fixes it then it eventuallt fubar's it. It's a small harness INSIDE the tranny.
Also, there is nothing wrong with a fluid change....just a fluid flush. Please don't get it flushed...that WILL wipe out a higher milage trans. Varnish builds up on the plates that takes up slack in the clutch pack, flusing it basically removes it.
Now mind you this does not happen to everyone. However, I have never heard of a fluid change killing a trans, but many stories of them crapping out soon after a flush...take it for what it's worth.