Kind of insulting, isn't it???
Every time I find a fired canine set with a fresh dump on or near the trap, I believe it has to do more with instincts urgencies, maybe a territorial thing to find some prominent object to mark the boundaries with...and my darkly dyed, fired trap is often the most prominent object sitting out in the immediate area. Could be a reaction to the food or gland attractors, too.
Who knows for sure. It happens. (sometimes I think it happens to me more than other trappers, but I could be wrong...)
I laugh to myself each time I find it happened at my sets. I guess if critters carried business cards maybe it would be less offensive if they left a nice, clean business card between the snapped jaws of my trap rather than just a fresh, steaming pile next to or even on the snapped trap itself.
Mostly it leaves me walking away trying not to take it "personally" or trying to keep my blood pressure lowered as I plan to avenge the critter with some new set nearby.