A quantitative study is made of the physical mechanisms responsible for the development of a collective hydrodynamic instability of a charged-particle beam in an isotropic plasma. The corresponding growth rate is calculated through an analysis of the dynamics of the motion of the beam particles in the field of their radiation. The coherence of the beam particles is responsible for a substantial amplification of the collective field excited by these particles. This field forms coherent bunches by the Veksler--MacMillan self-phasing mechanism. This review is addressed to specialists in plasma physics and microwave electronics.