Adiabatic fluctuation, of which inflation is the unique causal generator, begin with gravitational potential wells established in the very early universe. On the other hand isocurvature models, begin with no potential fluctuations. They are only generated as matter is moved around inside the causal horizon. As the fluid begins to compress, photon pressure resists the increase in the density perturbation, thereby allowing the gravitational potential to decay. Left in a highly compressed state, the fluid then oscillates as a cosine wave with enhanced amplitude.
Photon pressure resists the implied rarefaction of the fluid.
The fluid then falls back into the well, lending its self-gravity to enhance the depth. At the compressional maxima, photon pressure again causes potential decay leaving the fluid in a highly compressed state. However, here it drives a sine oscillation. Likewise adiabatic models could be reionized and lose their peak structure.