DONT MIX
beyond them being discharged, there is the reverse charge issue, which gets many consumers, because they just stuff some battery that fits into a device.
EVEN though the instructions on most of these devices, in the first part of the manuel with all the rest of the garbage nobody reads
it says, dont mix brands, styles, types, age, or drain ammount . well something like that.
when you use batteries in SERIES, you should use the same brand, the same type, the same package, the same ammount of use, if you mix up batteries in a drawer (we all do even me) then toss them in something, one could be WAY different capacity.
so ONE cell dies and the others keep going, and the one that died gets a reverse charge, and that can make it more Likly to leak.
so that is one factor, and is more likly to occur in Multicell lights (specially 4X or more).
AGE
a second factor is AGE, even if the cell is unused, there is toxic corrosive materials being contained to give you power. the Can that holds this stuff eventually breaks down over time. the more the manufactures shrink the can, to get more capacity, the sooner it can break down. then of course there is probably plastic linings and stuff, but all are effected by age. some manufatures push the edge of this containment envelope to far.
and over time , even without using the battery an old cell might be way low in capacity, which puts it back in factor 1 , one cell dies early.
RECHARGABLES
Most rechargable cells are better sealed, i have never had a rechargable damage a light unless i messed up something , meaning my fault.
TEMPERATURE
Temperatures, people USE this stuff, when they do it can sit in a 140*F car, it can get left in a -20*F garage, and temperatures can cause expansion and contraction of anything, loosening things up , or expanding them to failure.
RUST
water getting into your light, can increase the rate of oxidation,,, etc etc.
most all of this stuff is indicated on some packaging or spec sheet somewhere, but even if you do everything exactally right, stuff still fails, it just fails lots less.
The best thing you can do, is be a teeny bit more aware of all the things that can occur, and tell the "Family" not to mix up cells (takes 5 minutes), use rechargables, and ask the manufacture to repair or replace anything screwed over by thier product, so they Know what is important.
CHECK MORE
open up the light and check the cells say once every 6mo or so, it takes time to do the major damage, and the cells show inconsistancies before they ruin everything. look for changes in color on the wraper, Rust forming on the metal, or what looks like a liquid oil changing the coloration of the wraper.
DAMAGE
denting a battery , even from a hard drop on the floor, even a rechargable, even if you dont see the damage, an internal short can occur, which can discharge the battery or reduce the effectiveness of the seal.