The seismic wavefield depends on the velocity contrast between the country rock and the magma.  This contrast can change if properties of the magma change with time.  In a gas-charged magma, increasing pressure with depth reduces the volume of gas exsolved, increasing the seismic velocity with depth in the conduit. If there is a decompresion event, the decrease in the pressure allows the bubbles to grow further by diffusion.
 

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The diagram demonstrates how after a decompression event the volume of gas exsolved increases with time until all the gas that can be exsolved is exsolved.   There is a corresponding  decrease in the density, pressure and seismic velocity on the conduit until the equilibrium solution is reached.