So you need a natural process that makes simple sugars, and then creates a sugar syrup, and then dries that out to create crystals.
This is a bit of a stretch, but, here goes:
- Start with a planet with only bees and tiny, flowering plants.
- Bees produce honey from plants. (Honey is basically fructose, glucose and water)
- These bees overproduce and stockpile (honey lasts practically forever - so this isn't wasteful). They basically cover a decent percentage of the planet with storage hives filled with honey.
- Planet is subjected to intense heat (rogue star goes past?), killing everything with heat and fire, and evaporating the water from the honey, leaving a planet that's basically a sticky yellow goo under a powdered layer of dried honey.
- Planet dries and wind blows everything around - covering the planet in fine, sweet, yellow powder.
- Planet is hit by an ice asteroid, adding lots of heat and moisture. Planet is flooded with boiling water.
- In the turmoil everything gets mixed around and shaken up.
- As the water boils away, crystals are formed.
- The water is removed from the planet by solar wind over the aeons, leaving a perfectly dry environment.
- Wind scatters them around the planet.
I didn't realise sugar exploded at high temperature (I knew it would if aerosolized, like everything does, I didn't realise it would if heated). So with that in mind:
- The planet has a layer of heavy, unbreathable gas. Sulfur Hexafluoride, Radon, Carbon dioxide, etc. The nice environment with all the flowers and bees is up on highlands.
- Over the milenia, storage hives get lost in accidents and weather, and roll down into the lowlands. This results in a huge store of honey in a sterile environment.
- Heat pass still occurs, but the denser atmosphere of the lowlands keeps the temperature below 160 degrees, while the fertile highlands get burnt.
- The passing star fractures some geology, releasing boiling water from deep in the crust as geysers which flood the low plains with boiling water.
- As the water cools, sugar syrip is formed.
- As the water evaporates, the sugars become crystalised.
- Winds crack and spread the crystals around the lowlands of the planet.
- Everything settles down over the following milenia. The planets magnetic core cools, and all the water vapour and special gases get bled off into space from solar wind.
- If the air needs to be breathable in the lowlands, have the oxygen produced from geological processes, not plants. The less life on the planet the more plausible the sugar remains.