It's complex. There are many factors that can affect supply and any issue with any one factor will cause a shortage.
Propane is easily stored; that's not the issue.
With most petroleum products we use so much that there is simply not enough storage facilities for anything longer than about maybe 3 months supply. So all the propane available for sale (for example) is probably less than 3 months away from the wellhead.
If there is, for example, a refinery fire anywhere in North America that affects production, that's enough right there to cause a shortage.
Suppliers try to predict use but they can be wrong ... weather could be severe or mild, for example, and cause over or under supply. But that's just one factor ... there are dozens.
It starts right at the wellhead ... oil production is controlled partly by oil companies who own wells, but the majority is owned by tens of thousands of small producers.
Since oil underground is essentially available forever, a small producer will cut his output if the world price falls and has no incentive to over-produce at any time since he knows it will be worth more in one, five, thirty years.
That is why there is separate relationships between the world price and the pump price. As the world price falls, supply falls making pump prices rise.
Propane is part of that so your local price is all about available supply in your local (or regional) market.