1st off, plastic is a conductor, it's just a poor conductor, until you apply a few million volts. I could never understand why schools teach "inverse of conductor = insulator", make no sense. You have good and bad conductors, thats it.
2nd, the way pipes are buried can create a better opportunity for moisture to settle in that area along pipe path. This makes it easier for million+ volts to travel.
3rd, many AHJ's require tracer wire in trenches for various things. These tracers can essentially become a path for lighting, and many time they will turn into plasma, heat the surrounding moisture, and essentially explode your conduit and the stuff inside of it. I would not be surprised to learn that the scenario as described by OP has tracer wire in the trench along with the conduit.
Because you created the environment that way you should include pathways into the earth for the energy to follow. Stainless banding or clamps of the conduit and then tied to ground rod (every so many feet) will help direct the current down into the earth.