A gun is a poor storage place for ammo. I've had ammo fail in less than a year, if left continuously in the gun. Personally, I'd conjecture the powder gets rattled out of the primer. But however it happens, ammo failure, IME, has been primarily caused by it's constantly being in the gun. Shoot your carry ammo, after a couple months. One guy I know kept the same ammo in his carry guns since the 70s. When his nephew and I went to shoot these old revolvers, we cycled 11 rounds, repeatedly, between both guns, and every single round failed to discharge.