Sometimes there's a "jimp" along the rim of the case mouth. I'm not familiar with the Redding seater die, but I suspect that it is high quality. ...which means tight tolerances. I have a similar seating die with a sleeve which travels with the case mouth up as the bullet is pushed into the case mouth. If a "jimp" in the case mouth catches on that "tight tolerance" sleeve, the mouth never enters the sleeve properly, and when you lever it home, the case mouth gets crunched by the sleeve. I suggest a first step of neck sizing (even a partial neck size) of the brass to round out any imperfections on the neck rim. Suggest backing your sizing die off a full turn or more as all you want to do is work the rim of the case mouth, right? Although not conclusive, a look at the photo indicates to me that the "crunched" case is shorter than the "un-crunched" case, which means that something in your process (like the sleeve), didn't fit the way it should. I have had this happen with the sleeve, but I've also had it happen if I inadvertently slipped a 6.5 bullet into my .257 die!! Other than that, it is unlikely that it happened due to a bullet fit, or a crimp fit as that would have happened consistently ...not just once. I further recommend Hornady's bullet puller!! I regret to say that I use it a lot when I "change my mind" on something. 
