As far as I understand, this is all about resonance. When in the presence of EM radiation of the right frequency, an atom will absorb and then emit that frequency. This is much the same as with a quartz crystal (but not in the quantum domain). Most transitions are not particularly well defined; in general, the f in the hf is not easily reproduced accurately enough. The particular transition that's chosen for the standard is easy to use (practical reasons) and to reproduce accurately. This is a general principle for choosing any standard for a unit. The atoms can be kept in an environment that gives a consistent value of energy change between the two chosen states. There are many possible choices for standards; this particular one has been chosen.Why would an electron in caesium-133 undergo a transition of hyperfine states from a higher to a lower energy state, so as to emit energy?