Why is (tri-)ammonium phosphate ((NH4)3PO4) so unstable?
Ammonium phosphates are solids between 0 and 75°C. Because ammonium
  hydroxide $(\ce{NH4OH})$ is a much weaker base than the common metal
  hydroxides, ammonium phosphates are comparatively unstable.  Both
  triammonium phosphate ($\ce{(NH4)3PO4}$) and the double salt
  ($\ce{(NH4)3PO4.2(NH4)2HPO4}$) are unstable at room temperature and
  evolve ammonia to form diammonium phosphate. Even the commercial mono-
  and diammonium phosphates exhibit an ammonium vapor pressure in the
  solid form and in solution. Although MAP is more stable than DAP, it
  decomposes at high temperature to give ammonia and polyphosphoric
  acid. Diammonium phosphate decomposes to give ammonia and monoammonium
  phosphate at around 70°C.


Source: (2005) Ammonium Phosphates and Ammonium Sulfate. In: Synthetic Nitrogen Products. Springer, Boston, MA. DOI: 10.1007/0-306-48639-3_12.