I suspect poor annealing of that case. I don't know if Winchester double anneals a straight walled case like a .458. On cases with shoulders they do.

If you are going to reload a .458 Winchester, I would recommend that you anneal every time you size the brass. Lack of neck tension will cause you a lot of grief. I strongly suspect that brittle case necks (and subsequent poor neck tension)in this calibre were the cause of many of this cartridges (overblown) problems.

I load this calibre. When .458 brass was scarce in Canada I found a couple of tricks that helped extend the life of the brass and cartridge performance.

I do not expand the case at the mouth to allow for easier seating. I use a .50 calibre VLD inside neck chamfering tool to allow for problem free bullet seating. Seven twists of the wrist works. A dozen different bullet styles ( at least) played with , and no problems, AKA crushed cases.

I anneal every time. This ensures proper neck tension. The Hornady kit works. There are other ways of doing this.

I use a Lee factory crimp die. I seat and crimp in 2 passes. But I have never had a problem. Many times, my loads are varmit rifle accurate. I also use 2 seaters. One for the solid, and one for the soft.