Winchester casings splitting, anyone seen this before?

Yes, generally in more cheaply formed cases (i.e. HXP .30-06). The split starts life as an inclusion in the coin shaped disc, and is stretched into a line during forming.

I'd guess that production pressure has caused a few leaks in QC.

Hydrogen embrittlement from nickel plating*, nicked discs (as noted above) , poor headspacing (but this usually produces a stress ring near the head), overload, wrong chamber (e.g., Arisaka rifle), blank drawn too thin in impact extrusion.. etc.

Etc. Or a combination of factors. That's why there's a "Murphy's Law."

See also Hatcher's Notebook, page 206 ff.

As Hatcher once remarked, "There's never a dull moment in ammunition manufacture."

And let's face it. That's why there's a gas release hole in the receiver of almost every rifle.

Contact the mfr for a mailing label; I'm sure they'll be interested n it no matter what the cause.

Terry, 230RN

*Most plating operations involve plating out two cations: the intended metal and lots of hydrogen.

Yeah
BTDT with the Winchester
I was glad it was an extra stout firearm

Happens quite often, especially
with very much reused brass but
not necessarily.

Happened to me with new factory ammo.
Same firearm digests reloads with Remington brass without a hiccup.
My problem was just crap Winchester product

I suggest you call Winchester customer service. (They normally answer their phones.) Or write / email them through the Winchester website.
It has been my experience that they want to know about issues/problems with their products. I would venture a guess that they will provide a RMA and ask that you return the fired and unfired rounds to them and that they will send you replacement ammo and probably a discount coupon or several for use for the future purchase of Winchester products.

I would definitely contact them.

In my case, I already did all that with the
Winchester people, and they did send a
box and took it back. I never heard anything
else from them on the matter after we settled up
Had to have been defective cases
Never had the slightest problem with the
Remington or Starline brass
Oh well. .

Yeah, trouble is that it seems to be happening a lot more recently. Also not all with one manufacturer either. This seems to go hand in hand with pushing out as much product as possible. Smacks of QC problems to me.