What do you think cause this?

Range pick up - 9mm Aquila case.  NOT a step case as I originally thought.

Outer "ring" - deep groove.  Similar groove inside.  Obviously a case ready for case head separation.  The question is what caused this in the first place?

Internal view

Factory over aggressive crimp bullet groove

Interesting.  Could be.  Seems way off as I have yet to see another one.

LOOKS LIKE THE BASE IS SWOLLEN. OUT OF BATTERY FIRING? JP

That's a groove put into the brass to keep the bullet from going too deep into the case.  You find it with various calibers from some manufacturers.  The only problem you get with them is when you load a longer bullet and try to use the space below that line.  It makes a bulge in the Final product.

Pistol brass doesn't stretch and fail like rifle brass.

Cannelure

https://en.wikipedia.org/wiki/Cannelure

Very common on factory match wadcutter ammo. e.g. Federal Gold Medal Match, R-P, WW, etc.

Perhaps a factory goof.  Another possibility is that someone cut this case down for reloading as a 9 x 18 Makarov, then added the crimp to distinguish it from regular 9mm.  How long is the case?

Probably a one-off as I found another Aquila case in the same batch that does not have any marks on the case so not cannelure.

Case is not swollen or lengthen as it sits perfectly in a case gauge.

One thing about it being an aggressive crimp bullet grove is there is a concave groove both inside and outside the case.  I would think that an aggressive crimp would cause a concave groove outside and a convex groove inside?

Looks to me like the case just stretched away from the base, thinning the case in that area.  Of course the case is not too long as per the case gauge test but it could have been trimmed by its previous owner?

It is actually typical for Aguila brass.  I've seen it on 9mm cases as well.  Although there we some rumors that Aguila factory in Mexico was setup by Remington, I do not myself nor I do recommend this brass for reloading.
AP

Go to the aguilaammo.com site, goto the center fire pistol page, and look at the picture of the 9mm 124 grain ammunition.

Good call, you are absolutely right about this.  Crazy how hard that factory crimp is....