Earthquakes resulting from hydraulic fracturing (HF) can have delayed triggering relative to injection commencement over a varied range of time scales, with many cases exhibiting the largest events near/after well completion. This poses serious challenges for risk mitigation and hazard assessment. Here, we document a high-resolution, three-dimensional source migration process with delayed mainshock triggering that is controlled by local hydrogeological conditions. Our results reveal that poroelastic effects might contribute to induced seismicity, but are insufficient to activate a non-critically stressed fault of sufficient size. The rapid pore-pressure build-up from HF can be very localized and capable of producing large, felt earthquakes on non-critically stressed fault segments. We interpret the delayed triggering as a manifestation of pore-pressure build-up along pre-existing faults needed to facilitate seismic failure. Our findings can deepen our understanding of the current stress state of crustal faults and also explain why so few injection operations are seismogenic.