Interactions between fuel ash and bed material, which lead to agglomeration, are impeding a widespread use of alternative biomass fuels for energy conversion in fluidized-bed boilers. Reactions of certain ash components (alkaline metals in the case of biomass) with silicon of the quartz sand, commonly used as bed material, have been identified as main drivers for this phenomenon. Still, there is no consensus on the detailed mechanisms causing the attachment of ash on bed particles and the consequent growth of agglomerates. Only through a better understanding of these processes will it be possible to cast precise predictions and implement effective countermeasures for agglomeration issues. This paper reviews the current state of knowledge and delivers new insights into the mechanisms of coating-induced agglomeration. In particular, the influence of heterogeneous reactions on coating formation and the metamorphism of the coatings during agglomeration are investigated. The results are derived from experiments performed on lab-scale reactors described in our earlier publications. In addition, coated particles and agglomerates from large-scale plants or such extracted during the lab-scale experiments were evaluated using scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analyses. The findings are summarized in an updated description of the agglomeration process, eliminating heterogeneous chemical reactions from the major influencing factors and proposing a hypothesis for the behavior of the typical two-layer coatings during agglomeration.