The largest-magnitude slow slip events (SSEs) in the world are found to invade the seismogenic zone in the Guerrero seismic gap in Mexico. The Guerrero SSEs repeat within the same region every 4 yr. Here we examine the geological and geophysical evidence for the mechanism that allows for transient slip in the gap and at no other point along the >1000-km-long subduction zone. We find that long-lasting past magmatic activity most likely produced an impermeable gabbroic layer in the lower crust within the gap. This body has acted as a seal to trap fluids and over-pressurize the plate interface, thereby generating a transient slip region allowing for SSEs to invade the seismogenic zone. This suggests that transient slip can occur in typically seismogenic regions, but requires special conditions. It also adds to the growing evidence that the Guerrero gap may have a lower seismic hazard compared to the rest of the coast, which is opposite to what is reported by government agencies.