1. Phosphate can be introduced into the tank via food.
2. Phosphate bound in the tank can also be released into the water. As I undertand it the rock and sand can sequester phosphate and this bound phosphate can be released when phosphate in the water is reduced.
3. Phosphate is a 'building block' of life and can be released into the water as life forms die and aren't removed (fish, inverts, corals, including hitch hikers).
4. Phosphate (since it is contained in food) is present is fish and invert waste mater.
5. If you're using tap water, it may be introduced during water changes.