Q: Why do unstressed components sometimes fail for no obvious reason?
A: Sometimes they die of old age, sometimes the stress is there, but, as you say, it is not obvious.
"Old age" in a component is cumulative degradation due to physical or chemical changes. It is well-known that electrolytic capacitors and some types of film capacitor eventually die as a result of chemical reactions in their dielectric caused by a combination of trace impurities (oxygen, among others) and electrical stress. As integrated circuit structures follow Moore's Law and become smaller, the risk of dopant migration at normal operating temperatures causing failure within decades rather than centuries does increase; and mechanical fatigue in inductors due to fatigue arising from magnetostriction is a well-known effect. Some types of resistance material oxidize slowly in air, and more quickly as the air becomes more humid 1. Nobody expects batteries to last forever, either.
So when choosing components it is well to understand their structures and the possible age-related failure mechanisms which may operate even when the device is used under ideal conditions. This column is not the place to discuss such mechanisms in detail, but most reputable manufacturers will understand the aging of their products and are usually prepared to discuss service life and potential failure mechanisms - and many system manufacturers have publications on the safe service life of their products and the mechanisms limiting it 2.
However most electronic components can be expected to last for many decades, or even longer, under the correct operating conditions - but some of them still die. The reason is often unconsidered stresses.
As we continuously remind the readers of this RAQ column, one of the more useful formulations of Murphy's Law is "The Laws of Physics don't stop working just because you're not paying attention". Many stress mechanisms are easily overlooked.