When does your state budget come up? A month or two before that happens, they go on a spending spree to get rid of all the money that they have in their budget so they can say that they need more money for the next year. During this spending spree, they start more road projects then any other time in the year. Here, it started over the summer, about the end of July, beginning of August. When that happens, all road building materials jump in price because the government agencies demand certain mixes and grades of materials that cost more to create. I'm sure there is a lot of corruption in this too, but in my experience for just buying road grade gravel, it's a big jump in price because the quarry gears up to meat their demand and quantities they require. As a small time consumer, I'm not too important to all those multi million dollar contracts.
Once all the road work is done, things slow down, and prices adjust back down to normal. So in my area, late Fall through early Spring are when you get the best prices.
