The dynamics of Demand-Supply has been one of the prime factors affecting prices of Agro commodities but on a bigger picture currency movement can make upside-down changes in commodity prices.
By currency here we talk about the US Dollar. Normally there is an inverse relation between Value of dollar & commodity price that simply states when the dollar is strong the prices of commodities fall on the other hand if the dollar is weak the prices of commodities is seen to rise in other words since a long time their price tends to drop when the dollar strengthens against other major currencies and when dollar weakness against others, prices move higher.
Since USD is the benchmark pricing mechanism for most commodities thus, its value influences the prices of agro goods around the world.
Nevertheless, agriculture production differs from country to country eg. Majority of Corn and Soybean come from the USA, Asian countries produce most Spices while Cocoa is majorly produced in African countries. Thus, to standardize these variables US dollar plays the role by regulating prices and remains to be the fixed factor.
There are also short term and long term factors involved that affects trade and production but dollar plays one of the most major roles as due to its movement, price volatility is visible faster as the currency is the most stable forex instrument and other nations hold it as a reserve.
As value for USD fall buyer begins to have more buying power for commodities such as Corn, Wheat, Soybean because then it would require less currency to purchase each dollar and the extension of classic economic theory also comes to the action that when the price drops demand rise and vice versa.
The most significant variation in the correlation between UDS and commodity prices marked in decades was seen in the fourth quarter of 2016 when dollar in the index was trading at about 97 relatives to a basket of US trade partners currencies indicating strength but the price of commodities also remained strong, The Goldman Sachs commodity index moved 9% higher. History repeated itself in 2019 but the commodity market showed a typical inverse relationship again.
There are other factors too that govern the movement of prices in agro space such as government policies, weather & unforeseen events such as that are happening currently but it is not wrong to say that the correlation between US dollar and commodities is the most significant one.