To feed off-summit eruptions at volcanoes, magma moves by creating and passing through cracks that can propagate many kilometres downslope. Typically, these cracks are vertical (dykes). Here we show the propagation of a flat-lying magma-filled crack (sill) at Sierra Negra volcano, Galápagos Islands, using space-borne radar interferometric data spanning the 2018 eruption. This sill propagated along a 15-km-long curved trajectory, which is hard to explain with current understanding and models. We develop both a simple analytical analysis and a three dimensional (3D) numerical crack propagation model, which incorporates the effects of magma buoyancy, realistic topography and tectonic forces that may control the sill's propagation. We show that sill trajectories can only be understood and predicted if accounting for the interaction of all these factors, and explain the observed trajectory at Sierra Negra as the result of competing stresses being close to one another throughout the propagation of the sill. Under certain conditions, these events may be inherently unstable but remain predictable by combining high resolution observations with sophisticated theoretical understanding.