Bohr's third postulate states than an electron can make a transition from one stationary state of lower energy to another of higher energy if the required amount of energy is provided by means of a photon of appropriate energy (required to make the transition).
However does it say anything about why an excited electron will become de-excited? Why will an excited electron eventually come back to the ground state? After all it does have the energy to stay in the particular stationary state of higher energy. And by the definition of a stationary state it won't lose any energy in that state...