Just a couple of weeks ago, a guy here split a case in the extractor groove, not as bad as that. But it still blew the sideplate and the extractor off his Kahr CM9.
I do not recall his exact load but it was with 231 within handbook limits.
I think he had bullet setback against the feed ramp, which reduced the "combustion chamber" volume and increased pressure until the brass split.
Maybe you did, too.