I had a brand new LX508 and a two pixels died within 6 weeks of getting it. To cut a long story short, Pioneer told me (not directly, 3rd part engineers on behalf of pioneer) that they would not replace anything.
I went nuts, and if it wasnt for the relationship with my dealer, I wouldnt have a 5090 now. They (audio ex) replaced the screen for me to the new 5090, inc corresponding speakers and stand.
I was disgusted at pioneer, and their afterservice. Audio Ex I cannot fault and still to this day spend ££££'s with them for this reason, and never buy 'cheaper' online. IMO its just not worth it.
I was told that pixels dying are natural of plasma technology, and that unless (i forget how many) so many pixels die within a square inch of the screen then it is not classed as a fault


I dont know why they die, but I do know you are lucky to have been offered a repair. They will be there in a month, so dont take any crap, and get that offer from them in writing. Get it done asap, and make sure all your warranties are up to date afterwards.
Sorry to hear, I feel for you because I went through the same thing