It has only happened once and its only twice fired?

It would help to know what caliber and brand of cases you are using,
most of my experience with split necks is from over working the bras and making it work hardened. an over size chamber neck in some military rifles and on and on.

I have a 'factory wild cat' caliber. It's a 303-25 and the 'factory' ammo is made from necked down 303 Brit cases. Well, they split on firing! So, I took my 'new' cases and annealed them and so far, no more split necks. Apparently, work hardening is not the only 'hardening' mechanism. Old military ammo sometimes splits before firing. Age hardening takes place too. Chemical attack may also play a role.

It happens. It is just that the brass that was struck at the mill had certain defective composition. Most pieces will last many shots,
then you find splits in a few.
I find WW brass to have this annoying feature.
Rarely, to I see this with RP brass. Never, with Norma or Lapua brass.
I have been working with the WSSM"s. That has been a real problem. I see five or six pieces of a hundred that are split in the neck as new. Some fire seven or eight times without a split.
When I started reloading some 45 years ago, I would find splits in my cases sometimes.
Of course, not having much money, would fire the cartridge, just one more time, then discard it. I never had a problem with this, but will never do it now.
Case splits are a way of life. Discard them.

quote:

Old military ammo sometimes splits before firing. Age hardening takes place too. Chemical attack may also play a role.

Cartridge brass is pretty much the same allow no matter who stamps the cases. Each maker has their own process of annealing, including how many anneals they get and where in the process it occurs. Under war production schedules, and expected life of the ammo being produced, led some makers to skip some annealing. That left the brass a little to hard to withstand the long term stresses encountered in unexpectedly long storage. It wasn't "age" hardening, as such, that led to the splits in storage, it was simply that the necks could only withstand the stesses left from manufactoring for a finite time.

Exposure to ammonia vapors may weaken stored brass but that's an uncommon event.

I suspect the .303 brass used in the splittly cases was amongst those that got shorted in productions. Anneal it before AND after reforming and all should be well.

Believe it or not; Brass is in tension after
it is necked down. That is why it splites open
to relieve the tension. Resizing has the same effect to a lesser degree.
The easiest way to anneal that I have found is to hold the case head by my left thumb and
finger while heating the neck with a propane torch. When it gets hot dip the neck in water.
Good luck!