When that doesn't hold true you will end up with Failure To Deliver (FTD) which causes Very Bad Things™.

Generally speaking though commodities markets aren't rigged in the way that people complain about in some other markets but they are subject to pretty violent volatility in the face of uncertainty.

This is largely due to pretty fixed supply with highly variable demand. i.e I can't just shut off all the oil wells right now and return ships currently on-route to energy exchanges and even if I could I wouldn't have the storage. Storage is also an interesting one because storage costs money and was pretty much why oil contracts went negative for a short period.