Its the consumption of of water. One of the plants that is likely targeted by the tax is alfalfa, which is a plant used for animal feed. The top importers of alfalfa is china, uae and saudi arabia. Alfalfa is a very water intensive plant and most of its grown in the southwest/California.
It basically makes states $ at the cost that its a huge water drainer. Albeit not Arizona, keep in mind in california, resident use of water is only like 5% of the supply, and wouldnt be suprised of Arizona would be similar. Were in a drought basically to subsidize the price of poultry/cattle for other major nations