I think Bob Greenleaf, formerly of Savage, did an article on this in Handloader several years ago. He was of the opinion that it was due in part to erosion of the throat, causing the bullets to grab, then skip, then grab again (paraphrasing, obviously) which was causing pressure spikes, sometimes using wholly appropriate powder charges and bullets, just in highly eroded bores. It was a very interesting article, but I gave it to a cousin who was having difficulties with a worn .243 at the time, so I can't access it.

You can roll a turd in peanuts, dip it in chocolate, and it still ain't no damn Baby Ruth.