Khome,
As an economically minded person, this interests me greatly.
If you look at the chart for supply, and it shows a sudden spike in a single month, then it is probably a player dumping reserves.
If the chart shows a gradual increase, then production is larger than consumption for those products, leading to a gradual oversupply.
I'd look into it myself, but I don't have a presence on KB.
If the supply increase was gradual. Then from my analysis, it is caused by resent upgrades the the AI that controls C3 nations.
In the past, these Nations were a large net drain on global markets. Buying what they needed, but often failing to produce much in return.
The recent upgrade has enabled them balance their economies, so that they are actually producing enough to make up for their purchases.
Long term, this will be very good for Simcountry. The global markets will become real dynamic markets, rather than sitting in permanent severe shortages.
Signed President of DanNation on LU