Primarily while riding. Water is one of the major by-products of combustion. Since there is always some leakage past the rings you will get moisture in the case. The other thing is that as the engine heat-cycles from running to cold when it's parked the air/gas inside the case will contract drawing cool, damp air in from outside which will then condense on cold engine parts. Thing is, if the engine isn't run temperature changes and gradients are much less severe so the engine won't be drawing in or expelling such large quantities of air.
If you are parking the bike for winter? Change the engine oil, (The other ones will be fine.) fill the tank, remove the battery and store it somewhere temperate and every week or two give it a trickle charge, (This I am led to believe is a better option than a 'tender'.) and leave it alone. In spring, re-install the battery fire it up and ride.