The article mentions bronze production (thus indirectly mentioning copper and tin) not as a root cause, but rather a factor that spread the crisis from one region to another:

> What is clear is that once the collapse started, it was contagious, likely for two reasons: first that collapsing areas produced invading forces and refugee flows that destabilized their neighbors and second because as you will recall above, these states are interlinked and their rulers rely on trade to furnish the key military resource (bronze) as well as to acquire key prestige goods necessary to maintain the loyalty of the aristocracy.