I have an insanely dumb/gross question but..?

Every once and a while, when i walk into a public bathroom stalk, i saw that the person before me didnt flush (ill spare the details of what all was in it). This happens a lot more than you think would. I just wondered if any of you do not flush and what you're reason are? I seriously think its like a or a mental disorder for some people, but maybe im wrong. Sorry for the gross question but it boggles my mind that people would do that.