The reason the crystals start to form is because the honey at this extremely low moisture level can't really hold it's own for long. The sugar content is so high compared to the liquid it's dissolved in that it will actually start to separate. - same reason spring honey crystalizes
Put another way honey is a supersaturated liquid. Holding more sugar in a liquid form than it can sustain. Once a crystal of sugar precipitates out a cascade of crystallization occurs.
But the degree and rapidity of crystallization is influenced by the ratio of the types of sugars present from the floral nectar sources. Honey contains mostly glucose and fructose sugars with dextrose and maltose also occurring. Honeys high in fructose are very slow to granulate if at all while honeys high in glucose are quick to granulate.
This has long been known to beekeepers and generally tree honeys are slow to granulate while forbs are quick to do so.
Most notably Tupelo honey is claimed to never granulate while Canola, and all brassicas, is known to granulate in the comb even before the bees can cap it off.
Spun or creamed honey is nothing more than granulated honey produced by the beekeeper under controlled conditions to produce a granulated honey with extremely small crystals to avoid the coarse gritty texture of naturally occurring crystallization. Look up the Dyce Method for Spun Honey.
For liquid honeys that have crystallized they can be reliquified with gentle heat. Gentle as in low enough to avoid caramelizing or burning the sugar, just warm enough to melt the crystals. Depending on the water content of the honey crystallization can promote fermentation if the sugar and water separate. Cured honey in a liquid state has a water content to low to support fermentation but separation can allow fermentation to occur.