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In this paper experimental dependence of gap voltage collapse during vacuum breakdown on gap geometry and expanding plasma dynamics is investigated by breakdown development simulations. It is shown that experimentally observed differences in voltage fall time between a uniform field gap and a point cathode/plane anode gap is due to different perveances that these two gap configurations have during breakdown. For a point anode/plane cathode gap the braking of the cathode plasma emissive surface is responsible for much longer voltage collapse times. A large scatter in voltage fall time data is explained by the scatter in expansion velocity of the cathode plasma, which is caused by dependence of cathode plasma expansion velocity on the stochastic parameters of an initial cathode microplasma.