I'm not exactly sure how it works but the soot build up does periodically need to be burned off. That's where regen comes into play to burn the soot off of the filter. The truck idles high to do this and gets really hot. This is why you are seeing more and more trucks on fire on the side of the road. If the trailer is on fire, most likely brakes got too hot. If the fire looks like it started around the drive tires, again, most likely brakes. If the fire looks like it started at the nose of the truck and they were already parked somewhere, most likely a forced regen occurred around dry grass or some other flammable shit.