First of all, one of the main factors leading to increased phosphate demand is population growth. Asia is going through a strong period of population growth. People need to be fed. That's the main factor underpinning growing demand.
It's also related to upheaval in the supply chain. China was one of the leading importers from the Pacific countries. However, when the Chinese decided to keep their phosphate reserves for themselves, it upset the entire supply chain.
In short, growing demand stems in part from population growth and in part from a necessary realignment of the supply chain.