The July 2008 vulcanian explosion at Soufrière Hills Volcano (SHV), Montserrat, was preceded by one of the largest seismic swarms observed since the start of the eruption. We analyze the spectral and waveform properties of the earthquakes in this swarm and compare these observations to models of subsurface volcanic processes. We observe an initial volcano-tectonic (VT) swarm, followed by a large low-frequency (LF) swarm. We observe that the spectral content of LF events changes over time to carry more energy at lower frequencies. This shift to a lower frequency spectral content is concurrent with an increase in LF event rates. Ash-venting occurred a few hours before peak event rate. There was a subsequent increase in the higher-frequency energy component of LF events, concurrent with a decrease in event rates. Seismic quiescence occurred in the final 7 h before the vulcanian explosion. Our observations of VT seismicity are consistent with a model of decoupled gas ascent prior to magma emplacement. Changes in spectral properties and event rates suggest changes in conduit properties and/or pressure changes during magma ascent and stalling in the few days before the explosion. This interpretation is supported by previous petrological observations. Our analysis of repeating earthquakes suggests that hybrid and long-period events are part of the same source process and should not be considered separate classifications, at least at SHV. Our analysis highlights the potential of using simple spectral and waveform analyses for understanding changes in the magmatic system during transitions between quiescence and eruption.