I think the items in the Global Trade are more influenced by the random items required for the Cargo Ship and Airport cargo as much as anything else. We over produce shoes (or whatever they tell us is on order) because they don't give us the quantity ahead of time. Then I'm sure most of us then dump them on the GTHQ. If their algorithm of what is needed as a cargo order goes out to 10,000 cities in the same day, it will be flooded. Just a speculation.
I have noticed rhythms in what is needed to build residential zones as well. One day it will be Orange Slices in cargo, buildings and Air Port and then bags of flower, this would cause over producing as well - or demand for - then over producing.
It's all supply and demand.