Modern "alkaline" batteries use a liquid electrolyte which
incorporates a good deal of potassium hydroxide ("lye"), a corrosive
alkaline material. Over time, the electrolyte can attack the metal
shell of the battery, or the seals around the upper (positive)
electrode, and leak out.
I've read, in a couple of places, that this corrosive action is
relatively small if the battery is in its "as new" condition and has
not been partially discharged, and that the electrochemical processes
which occur when you draw charge out of the battery have the effect of
starting or speeding up the corrosion of the shell. This seems to
match my experience - alkaline batteries rarely seem to leak when
they're still in their packages, and leak rather more often once they
are partially or completely discharged.
Most equipment manufacturers recommend removing batteries from the
equipment if it's not to be used for a while, so as to reduce the risk
of equipment damage from leakage. If you can't do that - if you
really do need to have the equipment available "on standby" for rapid
use - then it's probably a good idea to swap out the batteries for
fresh ones any time you've used it significantly, and perhaps every 6
months in any case.
I've recently read one suggestion (by a ham-radio operator) that
battery-powered equipment which is used only occasionally, should be
powered by "heavy duty" carbon/zinc batteries rather than by
alkaline batteries. Carbon/zinc batteries have less power per cell
(roughly half as much for the "heavy duty" types), but they're less
expensive, and they use a chemistry with a near-neutral electrolyte
pH. If they "die of old age" it usually seems to be due to the
electrolyte drying out, rather than chewing its way out of its cage
and attacking innocent bystanders
Unfortunately the "heavy duty" carbon/zinc batteries seem to be almost
impossible to find in retail stores these days... everybody's
carrying alkalines.