Smokeless powder is a chemical compound, and one of the chemicals used in its manufacture is nitric acid.
When the right factors combine, (time, and storage conditions are two of them) the powder can chemically break down, releasing the nitric acid, which attacks the brass, making it brittle. So brittle it can crack just sitting in a drawer.
I have some 1897 6.5x55mm that did that. One case broke open at the shoulder, while on the stripper clip in the original box.
I think this accounts for most of what is generally called "season cracking" in old ammo. However there are cases of brass just appearing to crack from old age, so, it is apparently something that can happen from indeterminate causes, as well.
Oddly enough, some of the oldest rounds are the most stable. IF properly stored (throughout their lifetime). Black Powder is chemically more stable than smokeless, over time provided it doesn't get wet. It may become inert from moisture (again, environmental conditions of storage) but it doesn't break down chemically the way smokeless powder can.
The big problem with very old ammo is that other than uber rare cases, there is simply no way to know if the ammo has been properly stored throughout its lifetime. Generally, the closest we ever get to "cradle to grave" tracking is knowing the year and location where the ammo was made (IF we even know that...), with no records of the actual conditions it went through during decades and decades or a century or more, of storage.
In the early 70s, I fired a bunch of 1918 .30-06 and some .45ACP ammo. some of the 45 cases are still in my mixed brass stash, and a couple of the 06 cases I cut and sized into ,308 Win cases back in the 70s and I still have them in my brass stash too. Dark brown color that no amount of mechanical tumbling seems to change, but full functional, have been loaded multiple times without any failures.
Contrast this with some surplus 7.62NATO stuff a friend got about a decade ago, all of it from the late 90s. Fired 100rnds, 11 failed to fire, 17 of those that did had cases crack (body, not neck). Pulled down the rest for the bullets, where were fine, with a little surface discoloration where clumped powder, had stuck to the base of the bullet.