I can think of a few I've run into here are 3: 1) you are over a metal strip on the stub meant to protect wiring underneath. 2)there are no stubs, some construction materials are plastic, cardboard, styrofoam or simply not there defying good construction common sense. 3) duct work is behind wall. My neighbor is a master carpenter & does finishing work. We talk about crappy construction we see all the time. And without seeing your project it is impossible to get specific.
In addition to the preceding answers there is the possibility that the GFCI has simply worn out or become defective under load. Check the appliance by running it on an other GFCI. If it works fine...replace the GFCI.