Without more than a CHP report, I'm skeptical. I spent two and a half years on an M-109A1. I worked every crew position from grunt to chief.

155 rounds do not spontaneously explode. They require a trigger, specifically, a fuse. When I discharged in '77 there were three types of fuses, PD, proximity and timed, there are probably others now. Point detonating, the most commonly used, is self explanatory, it triggers the round when the point hits whatever. Proximity, rarely used because expensive, works on an echo principle and triggers at a certain distance above ground, 20 feet IIRC, used for maximum shrapnel spread. Timed, also self explanatory, triggers x amount of time after discharge, we only ever used those for flare rounds at night. I liked being the flare gun because you got to fire about every three minutes as opposed to the typical fire mission that took about twenty minutes or more from announced ready to fire.