Let us start with demand first
-a war will increase demand for weapons, armour, weapons grade metal and maybe other stuff
-expanding the city limits will create a demand for building materials
-a rise in the percentage of upper class will increase the demand for luxuries
- a change in the population percentage of citizens who have preferences for certain objects( more people with preferences for teacups, higher demand for teacups)
- Different species have dramatically different demands, so if one takes over another's site, the market will change.
- Lots of small villages nearby will tend to demand more staples. As cities grow in your area, more diverse and high end goods are relevant.
- Megabeasts dying = less demand for weapons and armor
- There could be climate change. Ice ages historically did sometimes swoop in in a matter of a few years, during the timeline of a fort game. Needs other changes first, like dwarves not ignoring weather until their fat melts, for starters...
- Overfishing/overgrazing/overforestry will encourage more of a natural colonial setup.
- You could code in fads and trends. Maybe whatever the current monarch likes is fashionable, so when they change around, the civ as a whole gets significantly different demands. And year to year clothing fashions, etc. (mostly high end, nobility wear)
- Fires and natural disasters require rebuilding.
- Droughts and famines change demand for foodstuffs.
- Disease changes supply for labor-intensive goods (too many people die, they can't make as many of these competitively) and alchemical products.