  Magnetic insulators can be characterized by a gap separating the singlet
ground state from the lowest energy triplet, S=1 excitation. If the gap can be
closed by the Zeeman interaction in applied magnetic field, the resulting S=1
quasiparticles, triplons, can have concentrations sufficient to undergo the
Bose-Einstein condensates transition. We consider macroscopic properties of the
triplon Bose-Einstein condensates in the Hartree-Fock-Bogoliubov approximation
taking into account the anomalous averages. We prove that these averages play
the qualitative role in the condensate properties. As a result, we show that
with the increase in the external magnetic field at a given temperature, the
condensate demonstrates an instability related to the appearance of nonzero
phonon damping and a change in the characteristic dependence of the speed of
sound on the magnetic field. The calculated magnetic susceptibility diverges
when the external magnetic field approaches this instability threshold,
providing a tool for the experimental verification of this approach.
