Everything wears out and the same goes for fillings and crowns. Over time the natural gum line changes and can recede away leaving the collar of the crown ( the area where the crown meets the natural tooth) exposed or uncovered.
When this happens our dietary saliva can become very acidic and start to erode the natural tooth structure away leaving a small space under the crown. This small space provides the perfect environment for sticky plaque and bacteria to accumulate and create decay.
It is not recommended to place a filling along the neck of the crown as there is no way to guarantee that all the decay has been removed, or that the underside has no moisture contamination which develops the possiblity that this pattern of this decay will come back.