Maybe it's an anomaly, but lately I've been replacing a lot of batteries in machines that have sat in storage for a month or more.
Modern farm equipment has a lot of built-in computers. Even with the key switch in the cab "off," those computers are pulling a few milli-amps of current all the time the machine is in storage. New, fresh batteries can usually withstand a steady micro-draw, but batteries that are more than halfway through their normal lifespan often can't deal with those never-ending micro-loads.
There are ways to avoid the dreaded "click-click-click" sound from the starter solenoid when you try to start a machine after it's been stored for a month or more. Start the machine once a month and let it run for an hour, and the battery should stay in good shape. Or, connecting a battery maintainer or float charger to the machine all the time it's in storage will keep the battery fully charged. Just be sure the battery maintainer/float charger is sized for the batteries it's connected to, and that the gizmo is DESIGNED TO STAY CONNECTED TO THE BATTERIES WITHOUT OVERCHARGING. A conventional "trickle charger" is not designed to stay connected to batteries, and will overcharge and damage batteries if left connected.
A third option is to unhook the batteries. Some newer models of combines and tractors actually come with battery disconnect switches. Onboard computers on those machines go to sleep without harm in the absence of power, and without any load, batteries maintain their charge longer. They will eventually self-discharge, which is normal, but should maintain a good charge for multiple months without need for recharging.
So, the next time you walk past the combine, big 4WD tractor, or self-propelled sprayer or mo-co that's been sitting idle for a month or more---hop in the cab and see if it will start. With big batteries often selling for more than $200 each, it's worth twisting the key every so often to keep batteries charged.