Andesitic-dacitic volcanoes share similar patterns of pre-eruptive unrest when they reawaken after centuries in repose. Thus (1) rates of volcano-tectonic (VT) seismicity follow restricted changes with time before bulk failure in the brittle crust, and (2) phreatic explosions are common before bulk failure. The VT trend suggests that magma ascends to the surface at the end of unrest, whereas phreatic activity suggests that magma had previously reached close enough to the surface to vaporize groundwater. The apparent contradiction in the timing of magma ascent is resolved using two historical case studies from Soufriere Hills volcano on Montserrat: a non-eruptive crisis in 1966 and a pre-eruptive sequence in 1995. The results suggest that pre-eruptive trends are driven by the pressurization of a leaky magma body in crust at near-lithostatic pore-fluid pressures; that the crust fails laterally over the magma body, rather than upwards towards the surface; and that the shallowest level for a pressurizing magma body coincides with when bulk failure opens a fracture large enough for magma to propagate dykes to the surface. The conditions provide the basis for distinguishing levels of increasing unrest and support the view that seeking deterministic constraints on forecasts of eruption is a realistic ambition.