You'll also notice that alkaline batteries stored in a pack usually don't leak, while ones stored in a flashlight have a much higher chance of leaking. I'd be interested to see if electron migration through the long connected leads is responsible. Another theory (my off-the-cuff theory) is that the battery begins a slow chemical or physical degradation after a single short usage. Perhaps test by putting batteries in a flashlight and resist the urge to click it on for a moment "just to check that they work".