  This paper theoretically analyzes the hydrodynamic shape stability problem
for sonoluminescing bubbles. We present a semianalytical approach to describe
the evolution of shape perturbations in the strongly nonlinear regime of
violent collapse. The proposed approximation estimating the damping rate
produced by liquid viscosity is used to elucidate the influence of the collapse
phase on subsequent evolution of the Rayleigh-Taylor instability. We
demonstrate that time derivatives of shape perturbations grow significantly as
the bubble radius vanishes, forming the dominant contribution to
destabilization during the ensuing bounce phase. By this effect the
Rayleigh-Taylor instability can be enhanced drastically, yielding a viable
explanation of the upper threshold of driving pressure experimentally observed
by Barber et al. [Phys. Rev. Lett. 72, 1380 (1994)].
