Low-frequency earthquakes (LFEs) have been observed in subduction zones and some major tectonic faults and may well be the most important constituents of tectonic tremors. In subduction zones, they were initially attributed to fluids released by dehydration reactions in downgoing slabs. Their seismic radiation pattern, however, is consistent with shear slip on the subduction interface, and this rapidly became the favored model. Recent studies indicate that the source duration of LFEs does not scale with magnitude, which can hardly be explained by shear rupture. We revisit the characteristics of LFE events in subduction zones as retrieved from local seismic arrays. We demonstrate that they can be explained equally well by forces acting in the direction of fluid motion. Such forces may be generated by a fast local pressure variations associated with unsteady fluid motion. The amount of fluid required for LFE activity is consistent with dehydration reaction rates. Plain Language Summary We discuss possible mechanisms of the low-frequency earthquakes, a component of the slow earthquake phenomena observed in fault zones, and suggest that they can be generated by very rapid fluid transients releasing the strong pressures gradients built during nonstationary fluid transport in the fault zones.